Photoelectric conversion device and imaging system

ABSTRACT

A photoelectric conversion device according to an exemplary embodiment includes a pixel which includes a photoelectric conversion unit and an amplifier transistor configured to output a signal generated by the photoelectric conversion unit. The photoelectric conversion unit includes a first electrode, a second electrode electrically connected to the amplifier transistor, a photoelectric conversion layer, and an insulating layer disposed between the photoelectric conversion layer and the second electrode. The photoelectric conversion layer includes quantum dots.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a photoelectric conversion device andan imaging system.

2. Description of the Related Art

Metal insulator semiconductor (MIS) photoelectric conversion deviceshave been proposed as photoelectric conversion devices used for imagesensors of cameras. In a photoelectric conversion device illustrated inFIG. 1 in WO2012/004923 (hereinafter referred to as “Patent Literature1”), a transparent electrode is disposed on top of a photoelectricconversion film, and a pixel electrode is disposed beneath thephotoelectric conversion film. An insulating film is disposed betweenthe photoelectric conversion film and the pixel electrode. PatentLiterature 1 describes enabling of correlated double sampling with theconfiguration described above, achieving noise reduction.

Japanese Patent Laid-Open No. 2011-228621 (hereinafter referred to as“Patent Literature 2”) discloses that a compound semiconductor, asilicon crystal, an ionic crystal, or the like is usable for aphotoelectric conversion film. Patent Literature 2 describes aphotoelectric conversion film formed of a material with a high opticalabsorption coefficient, such as amorphous silicon or an organicmaterial. According to Patent Literature 2, the use of such a materialleads to an increase in sensitivity.

SUMMARY OF THE INVENTION

A photoelectric conversion device according to an exemplary embodimentincludes a pixel which includes a photoelectric conversion unit and anamplification unit configured to output a signal generated by thephotoelectric conversion unit. The photoelectric conversion unitincludes a first electrode, a second electrode, a photoelectricconversion layer disposed between the first electrode and the secondelectrode, and an insulating layer disposed between the photoelectricconversion layer and the second electrode. The photoelectric conversionlayer includes quantum dots.

A photoelectric conversion device according to another exemplaryembodiment includes a pixel which includes a photoelectric conversionunit and an amplification unit configured to output a signal generatedby the photoelectric conversion unit. The photoelectric conversion unitincludes a first electrode, a second electrode, a photoelectricconversion layer disposed between the first electrode and the secondelectrode, and an insulating layer disposed between the photoelectricconversion layer and the second electrode. The photoelectric conversionlayer includes a first member, and a plurality of particles arranged inthe first member and having a particle size in a range of 1 nm to 20 nm.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of the configuration of a pixel of aphotoelectric conversion device according to a first exemplaryembodiment, and FIGS. 1B and 1C are diagrams illustrating equivalentcircuits of a photoelectric conversion unit of the photoelectricconversion device.

FIG. 2 is a schematic diagram of the overall configuration of thephotoelectric conversion device.

FIG. 3 is a diagram illustrating an equivalent circuit of a columncircuit of the photoelectric conversion device.

FIG. 4 is a schematic diagram of the planar structure of thephotoelectric conversion device.

FIGS. 5A and 5B are schematic diagrams of the cross-sectional structureof the photoelectric conversion device.

FIGS. 6A to 6F are schematic diagrams of the potential of thephotoelectric conversion unit of the photoelectric conversion device.

FIG. 7 is a diagram illustrating a timing chart of drive signals used inthe photoelectric conversion device.

FIG. 8 is a schematic diagram of the configuration of a pixel of aphotoelectric conversion device according to a second exemplaryembodiment.

FIG. 9 is a schematic diagram of the overall configuration of thephotoelectric conversion device.

FIG. 10 is a schematic diagram of the planar structure of thephotoelectric conversion device.

FIG. 11 is a schematic diagram of the cross-sectional structure of thephotoelectric conversion device.

FIGS. 12A to 12F are schematic diagrams of the potential of aphotoelectric conversion unit of the photoelectric conversion device.

FIG. 13 is a diagram illustrating a timing chart of drive signals usedin the photoelectric conversion device.

FIG. 14 is a schematic diagram of the configuration of a pixel of aphotoelectric conversion device according to a third exemplaryembodiment.

FIG. 15 is a diagram illustrating a timing chart of drive signals usedin the photoelectric conversion device.

FIG. 16 is a schematic diagram of the configuration of a pixel of aphotoelectric conversion device according to a fourth exemplaryembodiment.

FIG. 17 is a diagram illustrating a timing chart of drive signals usedin the photoelectric conversion device.

FIG. 18 is a schematic diagram of the configuration of a pixel of aphotoelectric conversion device according to a fifth exemplaryembodiment.

FIG. 19 is a diagram illustrating a timing chart of drive signals usedin the photoelectric conversion device.

FIG. 20 is a schematic diagram of the configuration of a pixel of aphotoelectric conversion device according to a sixth exemplaryembodiment.

FIGS. 21A to 21C are schematic diagrams of the structure of aphotoelectric conversion layer.

FIG. 22 is a block diagram of an imaging system according to a seventhexemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

Some exemplary embodiments may reduce noise.

The photoelectric conversion film disclosed in Patent Literature 1 orPatent Literature 2 may include multiple defect energy levels. This maylead to noise in the photoelectric conversion device disclosed in PatentLiterature 1 or Patent Literature 2.

For example, a dark current caused by a defect energy level may resultin noise. The presence of a defect energy level makes it difficult todeplete a photoelectric conversion film. Thus, when a signal is read outfrom the photoelectric conversion film, the photoelectric conversionfilm may not sufficiently be depleted. As a result, electric charge thatis not discharged from the photoelectric conversion film may remain inthe photoelectric conversion film. An electric charge remaining in thephotoelectric conversion film when a signal is read out from thephotoelectric conversion film may cause noise in the signal to beoutput.

Accordingly, some exemplary embodiments provide a photoelectricconversion device capable of reducing noise.

An embodiment of the present invention provides a photoelectricconversion device. A pixel included in the photoelectric conversiondevice includes a photoelectric conversion unit, and an amplificationunit that amplifies a signal generated by the photoelectric conversionunit. The photoelectric conversion device may include a plurality ofpixels. In this case, the photoelectric conversion device is, forexample, an image sensor. Alternatively, the photoelectric conversiondevice may include a single pixel. In this case, the photoelectricconversion device is, for example, an optical sensor. In FIG. 1A, apixel 100, a photoelectric conversion unit 101, and an amplifiertransistor 104 are illustrated by way of example.

The photoelectric conversion unit includes a first electrode, a secondelectrode, a photoelectric conversion layer disposed between the firstelectrode and the second electrode, and an insulating layer disposedbetween the photoelectric conversion layer and the second electrode.This configuration enables the photoelectric conversion unit toaccumulate electric charge generated by incident light as signal charge.Further, a signal from the photoelectric conversion unit can be read outby controlling the voltage to be supplied to a pixel circuit includingthe photoelectric conversion unit. In FIG. 1A, a first electrode 201, aphotoelectric conversion layer 205, an insulating layer 207, and asecond electrode 209 are illustrated by way of example.

The photoelectric conversion layer includes quantum dots. In FIGS. 21Ato 21C, quantum dots 10 are illustrated by way of example. The quantumdots are particles having a particle size in a range of, for example, nmto 20 nm. In some embodiments, the photoelectric conversion layerincludes a member formed of a material different from that of thequantum dots. The quantum dots are dispersed in the member. In someother embodiments, the photoelectric conversion layer includes a coverlayer that covers the quantum dots. In some other embodiments, thephotoelectric conversion layer includes a buried layer in which thequantum dots are buried. In FIGS. 21A to 21C, a member 11, a cover layer12, and a buried layer 14, which are formed of a material different fromthat of the quantum dots, are illustrated by way of example. These arecollectively referred to as “quantum-dot-disposed members”.

This configuration enables electric charge generated by photoelectricconversion to be accumulated in the photoelectric conversion layer. Whena signal is read out, the accumulated electric charge can be dischargedto the first electrode through a plurality of quantum dots.

The configuration in which the photoelectric conversion layer includesquantum dots may reduce the defect energy levels of the photoelectricconversion layer for the following reasons. Because the size of thequantum dots is small, the lattice defects in the quantum dots can bereduced. Consequently, noise reduction may be achieved.

In some embodiments, the material of the quantum dots is a compoundincluding at least one of the Group IV elements such as tin (Sn), lead(Pb), and copper (Cu), and at least one of the Group VI elements such asoxygen (O), sulfur (S), selenium (Se), and tellurium (Te). For example,the material of the quantum dots may be PbS, PbSe, PbTe, or CuO. In thisspecification, the compounds described above are referred to as “GroupIV-VI compounds”.

In some embodiments, the material of the quantum dots is a compoundincluding at least one of the Group III elements such as boron (B),aluminum (Al), gallium (Ga), indium (In), and thallium (T1), and atleast one of the Group V elements such as nitrogen (N), phosphorus (P),arsenic (As), and antimony (Sb). For example, the material of thequantum dots may be BN, GaAs, GaP, AlSb, InSb, InAs, or InGaAs. In thisspecification, the compounds described above are referred to as “GroupIII-V compounds”.

In some embodiments, the material of the quantum dots is a compoundincluding at least one of the Group II elements such as zinc (Zn),cadmium (Cd), and mercury (Hg), and at least one of the Group VIelements such as oxygen (O), sulfur (S), selenium (Se), and tellurium(Te). For example, the material of the quantum dots may be CdSe, CdTe,ZnS, or HgTe. The compounds described above are referred to as “GroupII-VI compounds”.

In some embodiments, a Group IV element such as carbon (C), silicon(Si), or germanium (Ge) is used for the material of the quantum dots.

The material of the quantum dots is preferably PbS, PbSe, PbTe, CdS,CdSe, CdTe, C, Si, Ge, or the like. This configuration may achievefurther noise reduction.

The material of the quantum-dot-disposed member is a semiconductormaterial, an insulator material, or a conductive material. Specifically,the material of the quantum-dot-disposed member may be selected fromcompounds containing PbSO4, PbO, PbSeO4, SiO, SiN, SiON, In2O3, sulfur(S), and sulfur (S), carbon (C), compounds containing carbon (C), andthe like. This configuration may improve sensitivity. The material ofthe quantum-dot-disposed member may also be selected from AlAs, AlGaAs,AlGaAs, GaAs, GaSb, AlSb, and the like. Alternatively, intrinsic silicon(Si) or low-concentration impurity-doped silicon (Si) may be used for amember in which quantum dots are dispersed. Alternatively, an organicresin in which quantum dots are dispersed may be used.

The band gap of the quantum-dot-disposed member is different from theband gap of the quantum dots. For example, the band gap of thequantum-dot-disposed member is larger than the band gap of the quantumdots. This configuration enables the formation of the potential forconfinement of carriers generated by photoelectric conversion in thequantum dots.

The quantum dots may have a larger band gap than a band gap in a bulkstructure made of the same material. Thus, preferably, a material havinga band gap equivalent to or smaller than the band gap of silicon (Si) isused for the quantum dots. Specifically, the band gap is preferably lessthan or equal to 1.12 electron volts (eV). For example, PbS, PbSe, PbTe,C, Si, Ge, or the like is preferably used. This configuration mayimprove sensitivity to light having a long wavelength, such as redlight.

As used herein, the term “the band gap of a material” refers to a bandgap in a bulk structure formed of the material alone. An example of amethod for measuring the band gap of a material used for the quantumdots will be described. First, a material used for the quantum dots isidentified. Then, a sample is produced by the same material as theidentified material, and the wavelength of light that the sample canabsorb is measured. The longest wavelength L of the wavelengths of theabsorbed light is converted to energy E according to E=h/L, where hdenotes Planck's constant. The obtained energy E corresponds to the bandgap.

A shell may be formed around the quantum dots. This may lead to improvedquantum efficiency. Since the size of the quantum dots is small, half ormore of the atoms in the quantum dots may be arranged on the surface ofthe quantum dots. The dangling bonds of the atoms arranged on thesurface may result in the formation of defect energy levels.Accordingly, multiple defect energy levels may exist on the surface ofthe quantum dots. The existence of multiple defect energy levels maycause recombination of electrons and holes to be more likely to occur,resulting in a possible reduction in quantum efficiency. Forming a shellaround the quantum dots may reduce the dangling bonds on the surface ofthe quantum dots. Consequently, quantum efficiency may be improved.

For example, the surface of the quantum dots of hydrophobic CdSe may becoated with ZnS to form a core-shell structure with a CdSe core and aZnS shell. Alternatively, the surface of the quantum dots of hydrophilicCdTe may be coated with a thiol compound. The methods described aboveallow electrons in excitons produced by absorption of light to beconfined in or near the core and to be less likely to be affected by thesurface.

Preferably, the lattice constant of the core is close to the latticeconstant of the shell. Specifically, it is desirable that the ratio ofthe two lattice constants be in a range of 0.9 to 1.1. Thisconfiguration may reduce lattice defects, resulting in noise reduction.

The quantum dots may include surface atoms which may be in thecoordination site. Such quantum dots have high reactivity, and are morelikely to induce aggregation. In order to stabilize the surface of thequantum dots, the surface of the quantum dots may be capped with acapping member. This configuration may prevent the quantum dots fromaggregating. Consequently, sensitivity may be improved.

The capping member is formed of a base covalently bound to metal atomsin the quantum dots, an organic polymer, or the like. For example,organic molecules having a straight-chain or branched aliphatichydrocarbon group with a carbon number of 2 to 30, preferably with acarbon number of 4 to 20, more preferably with a carbon number of 6 to18, are used. The capping member may have a functional group such as acarboxyl group, an amino group, an amide group, a nitrile group, ahydroxyl group, an ether group, a carbonyl group, a sulfonyl group, aphosphonyl group, or a mercapto group. Such functional groups facilitatecontrol of the coordination of the capping member. The capping membermay have a functional group different from a functional group forcoordination.

In addition, quantum dots having a particle size in a range of 1 nm to20 nm enable electrons to be subject to quantum confinement inside thequantum dots. This quantum confinement effect allows the quantum dots tohave a band gap which depends on the particle size. With thisconfiguration, the wavelength of the light to be detected can beselected in accordance with the size of the quantum dots.

In the following, exemplary embodiments of the present invention will bedescribed in detail with reference to the drawings. The presentinvention is not limited to the following exemplary embodiments. Amodification in which the configuration of the following exemplaryembodiments is partially modified without departing from the scope ofthe present invention also constitutes an exemplary embodiment of thepresent invention. Further, an example in which part of theconfiguration of any of the following exemplary embodiments is added toanother exemplary embodiment, or an example in which part of theconfiguration of any of the following exemplary embodiments is replacedwith part of the configuration of another exemplary embodiment alsoconstitutes an exemplary embodiment of the present invention.

First Exemplary Embodiment

FIG. 1A schematically illustrates a configuration of a pixel 100 of aphotoelectric conversion device according to this exemplary embodiment.The pixel 100 includes a photoelectric conversion unit 101, a resettransistor 102, a first capacitor 103, an amplifier transistor 104, anda selection transistor 105. While only one pixel 100 is illustrated inFIG. 1A, the photoelectric conversion device according to this exemplaryembodiment includes a plurality of pixels 100. In FIG. 1A, furthermore,the cross-sectional structure of the photoelectric conversion unit 101is schematically illustrated.

The photoelectric conversion unit 101 includes a first electrode 201, ablocking layer 203, a photoelectric conversion layer 205, an insulatinglayer 207, and a second electrode 209. The first electrode 201 isincluded in a node A illustrated in FIG. 1A. The second electrode 209 isincluded in a node B illustrated in FIG. 1A. The first electrode 201 isconnected to a voltage supply unit 110. The voltage supply unit 110supplies a plurality of voltages Vs to the first electrode 201 of thephotoelectric conversion unit 101. This configuration enablesaccumulation of signal charge in the photoelectric conversion unit 101and discharge of the signal charge from the photoelectric conversionunit 101. The discharge of the signal charge is carried out to read outa signal generated by the photoelectric conversion unit 101.

The voltage supply unit 110 supplies at least a first voltage Vs1 and asecond voltage Vs2 different from the first voltage Vs1 to the firstelectrode 201 of the photoelectric conversion unit 101. If signalcharges are holes, the second voltage Vs2 is a voltage lower than thefirst voltage Vs1. If signal charges are holes, for example, the firstvoltage Vs1 is equal to 5 V, and the second voltage Vs2 is equal to 0 V.If signal charges are electrons, the second voltage Vs2 is a voltagehigher than the first voltage Vs1. If signal charges are electrons, forexample, the first voltage Vs1 is equal to 0 V, and the second voltageVs2 is equal to 5 V. In this specification, the voltage on a groundednode is a reference voltage or 0 V unless otherwise stated.

The node B illustrated in FIG. 1A includes a gate of the amplifiertransistor 104. The amplifier transistor 104 is an amplification unit,and the gate of the amplifier transistor 104 is an input node of theamplification unit. That is, the second electrode 209 of thephotoelectric conversion unit 101 is electrically connected to theamplification unit. This configuration enables the amplification unit toamplify and output a signal generated by the photoelectric conversionunit 101.

The second electrode 209 is electrically connected to a first terminalof the first capacitor 103. In this exemplary embodiment, the firstterminal of the first capacitor 103 is included in the node B. That is,the second electrode 209 and the first terminal of the first capacitor103 are short-circuited. A second terminal of the first capacitor 103 isincluded in a node C. The second terminal is capacitively coupled to thefirst terminal. In other words, the node C is capacitively coupled tothe node B via the first capacitor 103. A predetermined voltage issupplied to the second terminal (the node C) of the first capacitor 103.In this exemplary embodiment, the second terminal (the node C) of thefirst capacitor 103 is grounded. That is, a voltage of 0 V is suppliedto the second terminal of the first capacitor 103.

A drain of the reset transistor 102 is connected to a node to which areset voltage Vres is supplied. A source of the reset transistor 102 isconnected to the second electrode 209 of the photoelectric conversionunit 101 and the gate of the amplifier transistor 104. Thisconfiguration enables the reset transistor 102 to reset the voltage onthe node B to the reset voltage Vres. That is, the reset transistor 102is a reset unit that supplies the reset voltage Vres to the secondelectrode 209. Turning off the reset transistor 102 brings the node Bconfigured to include the second electrode 209 of the photoelectricconversion unit 101 into an electrically floating state.

In this exemplary embodiment, a magnitude relationship between thevoltage Vs supplied to the first electrode 201 of the photoelectricconversion unit 101 and the reset voltage Vres is controlled toaccumulate signal charge in the photoelectric conversion unit 101 and todischarge the signal charge from the photoelectric conversion unit 101.The reset voltage Vres is an intermediate value between the firstvoltage Vs1 and the second voltage Vs2. For example, if signal chargesare holes, the reset voltage Vres is a voltage lower than the firstvoltage Vs1 and higher than the second voltage Vs2. If signal chargesare electrons, the reset voltage Vres is a voltage higher than the firstvoltage Vs1 and lower than the second voltage Vs2. In this exemplaryembodiment, the reset voltage Vres is equal to 3.3 V. The reset voltageVres is lower than a power supply voltage, and is higher than a voltageto be supplied to the grounded node.

A drain of the amplifier transistor 104 is connected to a node to whichthe power supply voltage is supplied. A source of the amplifiertransistor 104 is connected to an output line 130 via the selectiontransistor 105. A current source 160 is connected to the output line130. The amplifier transistor 104 and the current source 160 form asource follower circuit, and a signal generated by the photoelectricconversion unit 101 is output to the output line 130. A column circuit140 is also connected to the output line 130. A signal from the pixel100, which is output to the output line 130, is input to the columncircuit 140.

FIGS. 1B and 1C illustrate example equivalent circuit diagrams of thephotoelectric conversion unit 101. In this exemplary embodiment, thephotoelectric conversion unit 101 includes a photoelectric conversionlayer configured to accumulate signal charge, and an insulating layer.Accordingly, the photoelectric conversion unit 101 includes acapacitance component between the first electrode 201 and the secondelectrode 209. In the equivalent circuits illustrated in FIGS. 1B and1C, the capacitance component is represented as a second capacitor 111disposed between the first electrode 201 and the second electrode 209 ofthe photoelectric conversion unit 101. FIG. 1B illustrates an exemplaryembodiment in which the photoelectric conversion unit 101 includes ablocking layer. Thus, the blocking layer and the photoelectricconversion layer are illustrated using the circuit symbol of a diode112. FIG. 1C illustrates an exemplary embodiment in which aphotoelectric conversion layer does not include a blocking layer. Thus,the photoelectric conversion layer is illustrated using the circuitsymbol of a resistor 113. The structure of the photoelectric conversionunit 101 is described below.

FIG. 2 is a schematic diagram of an overall circuit configuration of thephotoelectric conversion device according to this exemplary embodiment.Portions having substantially the same functions as those in FIG. 1A areassigned the same numerals.

FIG. 2 illustrates 16 pixels 100 arranged in a matrix of four rows andfour columns. A plurality of pixels 100 included in each column areconnected to one output line 130. A row driver circuit 120 supplies adrive signal pRES and a drive signal pSEL to the pixels 100. The drivesignal pRES is supplied to the gates of the reset transistors 102. Thedrive signal pSEL is supplied to the gates of the selection transistors105. The reset transistors 102 and the selection transistors 105 arecontrolled by the drive signals described above. A plurality of pixels100 included in each row are connected to a common drive signal line.The drive signal line is a wiring line that transmits the drive signalpRES, the drive signal pSEL, and the like. In FIG. 2, signs indicatingrows, such as (n) and (n+1), are assigned to distinguish drive signalsto be supplied to different rows. The same applies to the otherdrawings.

FIG. 2 schematically illustrates the planar structure of the firstelectrodes 201 of the photoelectric conversion units 101. As illustratedin FIG. 2, photoelectric conversion units 101 of a plurality of pixels100 included in each row are configured to include a common firstelectrode 201. As described above, the voltage supply unit 110 suppliesa voltage Vs to the first electrodes 201. In this exemplary embodiment,a first electrode 201 is disposed for each row. Thus, the row drivercircuit 120 selects a row to which the voltage Vs is supplied from thevoltage supply unit 110. Signs indicating rows, such as (n) and (n+1),are assigned to distinguish voltages Vs to be supplied to differentrows.

In this exemplary embodiment, the configuration described above enablesa plurality of pixels 100 to be driven row-by-row.

The output lines 130 are respectively connected to the column circuits140. A column driver circuit 150 drives the column circuits 140 on acolumn-by-column basis. Specifically, the column driver circuit 150supplies a drive signal CSEL to a plurality of column circuits 140.Signs indicating columns, such as (m) and (m+1), are assigned todistinguish drive signals to be supplied to different columns. The sameapplies to the other drawings. This configuration enables signals readout in parallel for the respective rows to be sequentially output to anoutput unit 170.

The column circuits 140 will now be described in detail. FIG. 3illustrates an equivalent circuit of the column circuits 140 in the m-thcolumn and the (m+1)-th column. The column circuits 140 in the remainingcolumns are not illustrated.

A signal on each of the output lines 130 is amplified by a columnamplifier 301. An output node of the column amplifier 301 is connectedto a capacitor CTS via an S/H switch 303. The output node of the columnamplifier 301 is also connected to a capacitor CTN via an S/H switch305. The S/H switch 303 and the S/H switch 305 are controlled by a drivesignal pTS and a drive signal pTN, respectively. This configurationenables a noise signal, including reset noise, and an optical signalfrom each of the pixels 100 to be held. Accordingly, the photoelectricconversion device according to this exemplary embodiment enablescorrelated double sampling.

The capacitors CTS are connected to a horizontal output line 311 viahorizontal transfer switches 307. The capacitors CTN are connected to ahorizontal output line 313 via horizontal transfer switches 309. Thehorizontal transfer switches 307 and 309 are controlled by the drivesignal CSEL from the column driver circuit 150.

Both the horizontal output line 311 and the horizontal output line 313are connected to the output unit 170. The output unit 170 outputs adifference between a signal on the horizontal output line 311 and asignal on the horizontal output line 313 to an analog-to-digitalconversion unit 180. The analog-to-digital conversion unit 180 convertsan input analog signal into a digital signal.

Each of the column circuits 140 may be an analog-to-digital conversioncircuit. In this case, the analog-to-digital conversion circuit includesa holding unit that holds a digital signal, such as a memory or acounter. The holding unit holds digital signals into which a noisesignal and an optical signal are converted.

Next, the planar structure and cross-sectional structure of thephotoelectric conversion device according to this exemplary embodimentwill be described. FIG. 4 schematically illustrates the planar structureof the photoelectric conversion device. FIGS. 5A and 5B schematicallyillustrate the cross-sectional structure of the photoelectric conversiondevice. FIG. 4 illustrates four pixels 100 arranged in a matrix of tworows and two columns. The cross section illustrated in FIG. 5Acorresponds to the cross section taken along the line VA-VA in FIG. 4.The cross section illustrated in FIG. 5B corresponds to the crosssection taken along the line VB-VB in FIG. 4. Portions havingsubstantially the same functions as those in FIG. 1A are assigned thesame numerals. Note that, to indicate a transistor, a numeral isassigned to a gate electrode thereof. Further, a conductive memberforming a drive signal line is assigned the same numeral as a drivesignal supplied to the drive signal line. For example, a conductivemember assigned the numeral pRES forms a drive signal line for supplyingthe drive signal pRES.

The photoelectric conversion device includes a semiconductor substrate200. Various semiconductor regions, such as source regions and drainregions of pixel transistors, are disposed on the semiconductorsubstrate 200. Examples of the pixel transistors include the resettransistor 102, the amplifier transistor 104, and the selectiontransistor 105. Gate electrodes of the pixel transistors, and aplurality of wiring layers 202 including conductive members that formwiring lines are disposed on the semiconductor substrate 200. Thephotoelectric conversion units 101 are disposed on top of the wiringlayers 202.

As illustrated in FIG. 5A and FIG. 5B, the photoelectric conversion unit101 of each of the pixels 100 includes the first electrode 201 (commonelectrode), the blocking layer 203, the photoelectric conversion layer205, the insulating layer 207, and the second electrode 209 (pixelelectrode). The photoelectric conversion layer 205 is disposed betweenthe first electrode 201 and the second electrode 209. The blocking layer203 is disposed between the first electrode 201 and the photoelectricconversion layer 205. The blocking layer 203 is provided to preventelectric charge of the same conductivity type as the signal chargeaccumulated in the photoelectric conversion layer 205 from beinginjected into the photoelectric conversion layer 205 from the firstelectrode 201. The insulating layer 207 is disposed between thephotoelectric conversion layer 205 and the second electrode 209.

As illustrated in FIG. 2, the first electrodes 201 are electricallyisolated row-by-row. On the other hand, as illustrated in FIG. 5A, firstelectrodes 201 of a plurality of pixels 100 included in each row arecomposed of a common conductive member. For this reason, the firstelectrodes 201 are also referred to as common electrodes. The planarstructure of the first electrodes 201 are illustrated in FIG. 2, and thefirst electrodes 201 are not illustrated in FIG. 4.

As illustrated in FIG. 4 and FIG. 5A, the second electrode 209 of eachof the pixels 100 is electrically isolated from the second electrodes209 of the other the pixels 100. For this reason, the second electrodes209 are also referred to as individual electrodes. The blocking layer203, the photoelectric conversion layer 205, and the insulating layer207 are disposed continuously across the plurality of pixels 100. Thus,the blocking layer 203, the photoelectric conversion layer 205, and theinsulating layer 207 are not illustrated in FIG. 4.

As illustrated in FIG. 4, FIG. 5A, and FIG. 5B, each of the firstcapacitors 103 includes an upper electrode 211 and a lower electrode213. The upper electrode 211 and the lower electrode 213 face each otherwith an insulator interposed therebetween. This configuration provideshigh design flexibility in the capacitance value of the first capacitor103 for the following reasons. A semiconductor process such aslithography facilitates determination of the planar shapes of the upperelectrode 211 and the lower electrode 213. The first capacitor 103 mayhave any other structure. In another example, the first capacitor 103may be a PN junction capacitor having a larger capacitance value than apredetermined value.

Further, the upper electrode 211 and the lower electrode 213 of thefirst capacitor 103 are disposed in a wiring layer lower than the secondelectrode 209 of the photoelectric conversion unit 101. The upperelectrode 211 and the lower electrode 213 at least partially overlap thefirst electrode 201 or the second electrode 209 in plan view. Thisconfiguration can reduce the size of the pixel 100. Furthermore, each ofthe upper electrode 211 and the lower electrode 213 includes a portionthat does not overlap the reset transistor 102 or the amplifiertransistor 104.

In this exemplary embodiment, each of the first capacitors 103 is, forexample, a metal/insulator/metal (MIM) capacitor. Specifically, theupper electrode 211 and the lower electrode 213 are each composed of aconductive member such as metal. Alternatively, each of the firstcapacitors 103 may be a poly-Si/insulator/poly-Si (PIP) capacitor.Specifically, the upper electrode 211 and the lower electrode 213 areeach composed of polysilicon. Alternatively, each of the firstcapacitors 103 may be a metal oxide semiconductor (MOS) capacitor.Specifically, the upper electrode 211 is composed of a conductive membersuch as metal or polysilicon, and the lower electrode 213 is composed ofa semiconductor region.

As illustrated in FIG. 5A and FIG. 5B, the second electrode 209 of eachof the photoelectric conversion units 101 is connected to the gate ofthe amplifier transistor 104 via a conductive member 219. The secondelectrode 209 of the photoelectric conversion unit 101 is also connectedto the source region of the reset transistor 102 via the conductivemember 219 and a conductive member 220. Further, the second electrode209 is connected to the upper electrode 211 of the first capacitor 103via the conductive member 219. The lower electrode 213 of the firstcapacitor 103 is connected to a semiconductor region 217 via a contactplug 215. The semiconductor region 217 is grounded.

FIG. 5B illustrates the reset transistor 102 and the gate electrode ofthe amplifier transistor 104. A gate insulating film 230 is disposedbetween the gate electrode and the semiconductor substrate 200. Thesource regions and the drain regions of the pixel transistors aredisposed on the semiconductor substrate 200. Since the semiconductorregion 217 is grounded, the semiconductor region 217 may be electricallyconnected to a well 240 in which the source regions and drain regions ofthe transistors described above are formed.

The configuration of the photoelectric conversion unit 101 will bedescribed in detail. The first electrode 201 of the photoelectricconversion unit 101 is formed of a conductive member with a high opticaltransmittance. For example, a compound containing indium and/or tin,such as indium tin oxide (ITO), or a compound such as ZnO is used as amaterial of the first electrode 201. This configuration enables a largeamount of light to enter the photoelectric conversion layer 205. Thus,sensitivity may be improved. In another example, polysilicon or metalwith a thickness enough to allow a certain amount of light to passtherethrough may be used for the first electrode 201. Metal has lowresistance. Therefore, an exemplary embodiment in which metal is used asa material of the first electrode 201 may be advantageous to reducepower consumption or increase the driving speed.

The blocking layer 203 prevents electric charge of the same conductivitytype as that of the signal charge from being injected into thephotoelectric conversion layer 205 from the first electrode 201. Thephotoelectric conversion layer 205 is depleted by the voltage Vs appliedto the first electrode 201. Further, the gradient of the potential ofthe photoelectric conversion layer 205 is inverted in accordance withthe relationship between the voltage Vs applied to the first electrode201 and the voltage on the second electrode 209 (the node B). Thisconfiguration enables the accumulation of signal charge and thedischarge of the accumulated signal charge. The operation of thephotoelectric conversion unit 101 will be described below.

FIGS. 21A to 21C are schematic diagrams of the structure of thephotoelectric conversion layer 205 according to this exemplaryembodiment. In this exemplary embodiment, the photoelectric conversionlayer 205 includes quantum dots 10. The quantum dots 10 are made of PbS.The quantum dots 10 have the sufficiently compensated defect energylevels on the surface thereof. Thus, the defect energy levels in thequantum dots 10 and on the surface of the quantum dots 10 are reduced.The quantum dots 10 may be made of any of the various materialsdescribed above other than PbS.

In the example illustrated in FIG. 21A, the plurality of quantum dots 10are dispersed in a member 11 made of a material different from that ofthe quantum dots 10. In the example illustrated in FIG. 21B, thephotoelectric conversion layer 205 includes a cover layer 12 that coversthe plurality of quantum dots 10, and an intermediate layer 13 disposedbetween the cover layer 12 and the blocking layer 203 (not illustrated).In the example illustrated in FIG. 21C, the photoelectric conversionlayer 205 includes a buried layer 14 in which the quantum dots 10 areburied. The member 11, the cover layer 12, and the buried layer 14 areformed of Si, SiO, SiN, AlAs, AlGaAs, AlGaAs, GaAs, GaSb, AlSb, or thelike.

In this exemplary embodiment, ZnO is used for the blocking layer 203.Instead of this, an n-type or p-type semiconductor of the same kind asthat of the semiconductor used for the photoelectric conversion layer205 and having a higher impurity concentration than that of thesemiconductor used for the photoelectric conversion layer 205 may beused for the blocking layer 203. The positions of Fermi level differbecause of the different impurity concentrations. Thus, a potentialbarrier can be formed only for either electrons or holes. The blockinglayer 203 is of a conductivity type in which the majority carriers areelectric charge carriers of a conductivity type opposite to that of thecarriers of the signal charge.

Alternatively, the blocking layer 203 may be formed of a materialdifferent from that of the photoelectric conversion layer 205. Thisconfiguration enables the formation of a heterojunction. The band gapsdiffer because of the difference in material. Thus, a potential barriercan be formed only for either electrons or holes.

The insulating layer 207 is disposed between the photoelectricconversion layer 205 and the second electrode 209. The insulating layer207 is formed of an insulating material. For example, an inorganicmaterial such as silicon oxide, amorphous silicon oxide (hereinafterreferred to as “a-SiO”), silicon nitride, or amorphous silicon nitride(a-SiN), or an organic material is used as a material of the insulatinglayer 207. The insulating layer 207 has desirably a thickness enough toprevent tunneling of electric charge. This configuration can reduceleakage current, and can therefore reduce noise. Specifically, it isdesirable that the insulating layer 207 have a thickness greater than orequal to 50 nm.

In a case where a-Si, a-SiO, or a-SiN is used for the blocking layer203, the photoelectric conversion layer 205, and the insulating layer207, hydrogenation may be performed and dangling bonds may be terminatedby hydrogen. This configuration may reduce noise.

The second electrode 209 is composed of a conductive member such asmetal. The second electrode 209 is made of the same material as aconductive member forming a wiring line or a conductive member forming apad electrode for external connection. This configuration enablessimultaneous formation of the second electrode 209 and a wiring line ora pad electrode. Accordingly, the manufacturing process may besimplified.

Next, the operation of the photoelectric conversion unit 101 accordingto this exemplary embodiment will be described. FIGS. 6A to 6Fschematically illustrate energy bands in the photoelectric conversionunit 101. In FIGS. 6A to 6F, the energy bands of the first electrode201, the blocking layer 203, the photoelectric conversion layer 205, theinsulating layer 207, and the second electrode 209 are illustrated. Thevertical axis in FIGS. 6A to 6F represents the potential of electrons.The potential of electrons increases along the vertical axis in anupward direction in FIGS. 6A to 6F. Accordingly, the voltage decreasesalong the vertical axis in an upward direction in FIGS. 6A to 6F. Forthe first electrode 201 and the second electrode 209, the energy levelof free electrons is illustrated. For the blocking layer 203 and thephotoelectric conversion layer 205, a band gap between the energy levelof the conduction band and the energy level of the valence band isillustrated. The potential of the photoelectric conversion layer 205 atthe interface between the photoelectric conversion layer 205 and theinsulating layer 207 is referred to as the “surface potential of thephotoelectric conversion layer 205” or simply as the “surfacepotential”, for convenience.

In the operation of the photoelectric conversion unit 101, the followingsteps (1) to (6) are repeatedly performed: (1) the reset of the inputnode of the amplification unit, (2) the readout of a noise signal, (3)the discharge of signal charge from the photoelectric conversion unit,(4) the readout of an optical signal, (5) the reset before accumulationof signal charge is started, and (6) the accumulation of signal charge.In the following, the respective steps will be described.

FIG. 6A illustrates the state of the photoelectric conversion unit 101in step (1) to step (2). The first voltage Vs1 is supplied to the firstelectrode 201 from the voltage supply unit 110. The first voltage Vs1 isequal to, for example, 5 V. In the photoelectric conversion layer 205,holes plotted using empty circles are accumulated as signal chargesgenerated during an exposure period. The surface potential of thephotoelectric conversion layer 205 changes in the direction in which thesurface potential decreases (i.e., in the direction in which voltageincreases) in accordance with the number of accumulated holes. In thecase of accumulation of electrons, the surface potential changes in thedirection in which the surface potential increases (i.e., in thedirection in which voltage decreases) in accordance with the number ofaccumulated electrons.

In this state, the reset transistor 102 is turned on. Accordingly, thevoltage on a node including the second electrode 209, that is, thevoltage on the node B illustrated in FIG. 1A, is reset to the resetvoltage Vres. In this exemplary embodiment, the node B includes the gateof the amplifier transistor 104. Thus, the voltage at the gate of theamplifier transistor 104 is reset. The reset voltage Vres is equal to,for example, 3.3 V.

After that, the reset transistor 102 is turned off. Accordingly, thenode B is brought into an electrically floating state. In this case,reset noise (noise kTC1 illustrated in FIG. 6A) may be generated by thereset transistor 102.

The surface potential of the photoelectric conversion layer 205 maychange in accordance with a change in the voltage on the secondelectrode 209 during the reset operation. In this case, the direction inwhich the voltage on the second electrode 209 changes is opposite to thedirection in which the voltage on the second electrode 209 has changeddue to the accumulation of signal charge. For this reason, the holes ofthe signal charge remain accumulated in the photoelectric conversionlayer 205. In addition, the blocking layer 203 prevents injection ofholes from the first electrode 201. Thus, the amount of signal chargeaccumulated in the photoelectric conversion layer 205 does not change.

If the selection transistor 105 is in an on state, the amplifiertransistor 104 outputs a noise signal (Vres+kTC1) including the resetnoise from the pixel 100. The noise signal is held in the capacitor CTNof the column circuit 140.

FIGS. 6B and 6C illustrate the state of the photoelectric conversionunit 101 in step (3). First, the second voltage Vs2 is supplied to thefirst electrode 201. Since holes are used as signal charges, the secondvoltage Vs2 is a voltage lower than the first voltage Vs1. The secondvoltage Vs2 is equal to, for example, 0 V.

In this case, the voltage on the second electrode 209 (the node B)changes in the same direction as the direction in which the voltage onthe first electrode 201 changes. An amount of change dVB in the voltageon the second electrode 209 is determined in accordance with the ratioof the capacitance value C1 of the first capacitor 103 connected to thesecond electrode 209 to the capacitance value C2 of the second capacitor111 included in the photoelectric conversion unit 101. The amount ofchange dVB in the voltage on the second electrode 209 with respect to anamount of change dVs in the voltage on the first electrode 201 is givenby dVB=dVs×C2/(C1+C2). The node B including the second electrode 209 mayalso include other capacitance components. The other capacitancecomponents have a much smaller capacitance value than the capacitancevalue C1 of the first capacitor 103. Thus, the capacitance value of thenode B can be regarded as being equal to the capacitance value C1 of thefirst capacitor 103.

In this exemplary embodiment, the amount of change dVs in the voltage onthe first electrode 201 is much larger than the amount of change dVB inthe voltage on the second electrode 209. Thus, the potential of thesecond electrode 209 is lower than the potential of the first electrode201, and the gradient of the potential of the photoelectric conversionlayer 205 is inverted. Accordingly, an electron plotted using a solidcircle is injected into the photoelectric conversion layer 205 from thefirst electrode 201. In addition, some or all of the holes accumulatedin the photoelectric conversion layer 205 as signal charges move to theblocking layer 203. The holes that have moved are recombined with themajority carriers in the blocking layer 203 and disappear. Consequently,the holes in the photoelectric conversion layer 205 are discharged fromthe photoelectric conversion layer 205. For the depletion of the entirephotoelectric conversion layer 205, all the holes accumulated as signalcharges are discharged.

Then, in the state illustrated in FIG. 6C, the first voltage Vs1 issupplied to the first electrode 201. Accordingly, the gradient of thepotential of the photoelectric conversion layer 205 is inverted again.Thus, the electrons injected into the photoelectric conversion layer 205in the state illustrated in FIG. 6B are discharged from thephotoelectric conversion layer 205. On the other hand, the blockinglayer 203 prevents injection of holes into the photoelectric conversionlayer 205 from the first electrode 201. Accordingly, the surfacepotential of the photoelectric conversion layer 205 changes inaccordance with the number of holes that have been accumulated. Inaccordance with the change in surface potential, the voltage on thesecond electrode 209 changes from the reset state by a voltage Vpcorresponding to the number of holes that have disappeared. That is, thevoltage Vp corresponding to the number of holes accumulated as signalcharges appears at the node B. The voltage Vp corresponding to thenumber of accumulated holes is referred to as an “optical signalcomponent”.

In the state illustrated in FIG. 6C, the selection transistor 105 isturned on. Accordingly, the amplifier transistor 104 outputs an opticalsignal (Vp+Vres+kTC1) from the pixel 100. The optical signal is held inthe capacitor CTS of the column circuit 140. The difference between thenoise signal (Vres+kTC1) read out in step (2) and the optical signal(Vp+Vres+kTC1) read out in step (4) is a signal based on the voltage Vpcorresponding to the accumulated signal charge.

FIG. 6D illustrates the state of the photoelectric conversion unit 101in step (5). The reset transistor 102 is turned on, and the voltage onthe node B is reset to the reset voltage Vres. After that, the resettransistor 102 is turned off. In the way described above, the node B isreset before the accumulation of signal charge is started, enabling theoptical signal component for the preceding frame which has beenaccumulated in the node B to be removed. Accordingly, the dynamic rangemay be prevented from being narrowed in accordance with the accumulationof optical signals in the node B. Note that the reset beforeaccumulation of signal charge is started in step (5) may not necessarilybe performed.

Also in this case, reset noise (noise kTC2 illustrated in FIG. 6D) maybe generated by the reset transistor 102. The generated reset noise canbe removed through the reset operation in step (1) after the completionof the accumulation period.

FIGS. 6E and 6F illustrate the state of the photoelectric conversionunit 101 in step (6). The first voltage Vs1 is supplied to the firstelectrode 201, and the reset voltage Vres is supplied to the node B.Since the reset voltage Vres is lower than the first voltage Vs1, theelectrons in the photoelectric conversion layer 205 are discharged tothe first electrode 201. In contrast, the holes in the photoelectricconversion layer 205 move to the interface between the photoelectricconversion layer 205 and the insulating layer 207. However, the holesare not movable to the insulating layer 207, and are thus accumulated inthe photoelectric conversion layer 205. In addition, as describedpreviously, the blocking layer 203 prevents the holes from beinginjected into the photoelectric conversion layer 205. In this state,when light enters the photoelectric conversion layer 205, only the holesin the electron-hole pairs generated by photoelectric conversion areaccumulated in the photoelectric conversion layer 205 as signal charges.After the accumulation operation has been performed for a certainperiod, the operations in steps (1) to (6) are repeatedly performed.

The accumulated holes cause a change in the surface potential of thephotoelectric conversion layer 205. In accordance with the change insurface potential, the voltage on the second electrode 209 increases.This increase is represented by Vp0 in FIG. 6F. In the reset operationin FIG. 6A, as described above, the voltage on the second electrode 209changes so as to cancel out the change of the voltage Vp0. That is, thevoltage on the second electrode 209 decreases. Accordingly, the surfacepotential of the photoelectric conversion layer 205 changes in thedirection in which the surface potential increases.

If signal charges are electrons, the second voltage Vs2 is a voltagehigher than the first voltage Vs1. Thus, the gradient of the potentialillustrated in FIGS. 6A to 6F is inverted. The other operations aresubstantially the same.

In the operation described with reference to FIGS. 6A to 6F, thegradient of the potential of the photoelectric conversion layer 205 isinverted in the state illustrated in FIG. 6B, enabling the discharge ofthe accumulated signal charge. Non-inversion of the gradient of thepotential of the photoelectric conversion layer 205 causes theoccurrence of electric charge that is not discharged. Thus, noise mayoccur. Here, as the amount by which the amount of change dVs in thevoltage on the first electrode 201 is larger than the amount of changedVB in the voltage on the second electrode 209 (the node B) increases,the gradient of the potential is more likely to be inverted. That is, asthe amount by which the amount of change dVs in the voltage on the firstelectrode 201 is larger than the amount of change dVB in the voltage onthe second electrode 209 increases, further noise reduction isachievable.

As described above, there is a relationship represented bydVB=dVs×C2/(C1+C2) between the amount of change dVs in the voltage onthe first electrode 201 and the amount of change dVB in the voltage onthe node B. Modifying this equation yields the following equation forthe amount of change dVs in the voltage on the first electrode 201:dVs=dVB+(C1/C2)×dVB. That is, the amount of change dVs in the voltage onthe first electrode 201 is larger than the amount of change dVB in thevoltage on the second electrode 209 by (C1/C2)×dVB. Accordingly, as thecapacitance value C1 of the node B increases, the difference between theamount of change dVs in the voltage on the first electrode 201 and theamount of change dVB in the voltage on the second electrode 209increases.

In this exemplary embodiment, the first capacitor 103 is connected tothe second electrode 209. Thus, the capacitance value C1 of the node Bcan be increased. This configuration enables the amount of change dVs inthe voltage on the first electrode 201 to be larger than the amount ofchange dVB in the voltage on the second electrode 209. Consequently,depletion of the photoelectric conversion layer 205 is more likely to beachieved, resulting in a reduction in electric charge that is notdischarged. According to this exemplary embodiment, therefore, noisereduction may be achieved.

A description will be given of a comparative example in which the firstcapacitor 103 is not connected to the node B. In this configuration, thenode B has a capacitance that may include a capacitance component due toa PN junction in a semiconductor region and a parasitic capacitancecomponent of a wiring line. Since the capacitance components describedabove are negligibly smaller than the capacitance value C2 of the secondcapacitor 111 included in the photoelectric conversion unit 101, thevalue given by C1/C2 is substantially equal to zero. Thus, when thesecond voltage Vs2 is supplied to the first electrode 201, the amount ofchange dVs in the voltage on the first electrode 201 is substantiallyequal to the amount of change dVB in the voltage on the second electrode209. In this case, the gradient of the potential may not be inverted inthe state illustrated in FIG. 6B, resulting in a possibility that someof the holes accumulated as signal charges will not be discharged. Inthis exemplary embodiment, by contrast to the comparative example, theamount of signal charge that is not discharged can be reduced, resultingin noise reduction.

A description will now be given of the relationship among thecapacitance value C1 of the first capacitor 103, the capacitance valueC2 of the second capacitor 111 included in the photoelectric conversionunit 101, and the voltage supplied to each unit.

In this exemplary embodiment, the photoelectric conversion unit 101includes the blocking layer 203, the photoelectric conversion layer 205,and the insulating layer 207. The blocking layer 203 has a higherconductivity than the photoelectric conversion layer 205 and theinsulating layer 207. Thus, the capacitance value C2 of the secondcapacitor 111 included in the photoelectric conversion unit 101 is acombined capacitance of a capacitance component Ci formed by thephotoelectric conversion layer 205 and a capacitance component Cinsformed by the insulating layer 207. Specifically, the capacitance valueC2 of the second capacitor 111 is given by Expression (1) as follows:

C2=Ci×Cins/(Ci+Cins)  (1)

The capacitance component Ci and the capacitance component Cins arerespectively given by Expression (2) and Expression (3) as follows:

Ci=E0×Ei×Ss/di  (2)

Cins=E0×Eins×Ss/dins,  (3)

where Ss denotes the area of the second electrode 209 in plan view, didenotes the thickness of the photoelectric conversion layer 205, dinsdenotes the thickness of the insulating layer 207, Ei denotes therelative dielectric constant of the photoelectric conversion layer 205,Eins denotes the relative dielectric constant of the insulating layer207, and E0 denotes the dielectric constant of vacuum.

The fringing field around the second electrode 209 is substantiallynegligible. Thus, it is sufficient to take into account only the area Ssof the second electrode 209 in plan view as the area used for thecalculation of capacitance. The area Ss of the second electrode 209 inplan view is, for example, the area of the second electrode 209illustrated in FIG. 4. Further, in FIGS. 5A and 5B, the thickness di ofthe photoelectric conversion layer 205 and the thickness dins of theinsulating layer 207 are illustrated.

The capacitance value C1 of the first capacitor 103 is given byExpression (4) as follows:

C1=E0×Ed×Sd/dd,  (4)

where Sd denotes the area of the upper electrode 211 or the lowerelectrode 213 in plan view, dd denotes the distance between the upperelectrode 211 and the lower electrode 213, and Ed denotes the dielectricconstant of an insulating layer between the upper electrode 211 and thelower electrode 213.

In this exemplary embodiment, the voltage Vs on the first electrode 201(the node A) is controlled using the first voltage Vs1 and the secondvoltage Vs2 to accumulate signal charge and discharge the signal chargedue to the depletion of the photoelectric conversion layer 205. Thecapacitance value C1 of the first capacitor 103 and the capacitancevalue C2 of the second capacitor 111 satisfy the following relationship,thereby achieving a reduction in the electric charge remaining in thephotoelectric conversion layer 205 during the discharge of signal chargedescribed above. An exemplary embodiment in which signal charges areholes will be described first.

In the following, for simplicity, the capacitance value C1 of the firstcapacitor 103 is k times as large as the capacitance value C2 of thesecond capacitor 111. That is, the capacitance value C1 and thecapacitance value C2 have a relationship of Expression (5) as follows:

C1=k×C2  (5)

As described previously, the amount of change dVs in the voltage on thefirst electrode 201 and the amount of change dVB in the voltage on thesecond electrode 209 (the node B) have a relationship given byExpression (6) as follows:

dVB=dVs×C2/(C1+C2).  (6)

Expression (5) and Expression (6) yield Expression (7) as follows:

dVB=dVs/(1+k).  (7)

To accumulate holes as signal charges, it is desirable that the firstvoltage Vs1 and the reset voltage Vres satisfy a relationship ofExpression (8) as follows:

Vs1>Vres.  (8)

To transfer the holes of the signal charge, it is desirable that thefirst voltage Vs1, the reset voltage Vres, the amount of change dVs inthe voltage on the first electrode 201, and the amount of change dVB inthe voltage on the second electrode 209 satisfy a relationship ofExpression (9) as follows:

Vs1+dVs<Vres+dVB.  (9)

If the relationship of Expression (8) is satisfied, the gradient of thepotential that allows holes to drift toward the insulating layer 207 canbe formed in the photoelectric conversion layer 205. If the relationshipof Expression (9) is satisfied, it is easy to reverse the gradient ofthe potential of the photoelectric conversion layer 205.

Expression (7) and Expression (9) yield Expression (10).

Vs1−Vres+dVs<dVs/(1+k)  (10)

Here, k>0. Thus, Expression (10) is modified into Expression (11) belowby multiplying both sides of Expression (10) by (1+k).

(1+k)×(Vs1−Vres+dVs)<dVs  (11)

The amount of change dVs in the voltage on the first electrode 201 isgiven by dVs=Vs2−Vs1. Thus, Vs1−Vres+dVs=Vs2−Vres is obtained. In anexemplary embodiment in which signal charges are holes, the resetvoltage Vres is higher than the second voltage Vs2. That is, Vs2−Vres<0is obtained. Accordingly, the relationship of Expression (12) below issatisfied.

Vs1−Vres+dVs<0  (12)

Accordingly, dividing both sides of Expression (11) by (Vs1−Vres+dVs)changes the orientation of the inequality sign, yielding therelationship of Expression (13) as follows:

1+k>dVs/(Vs1−Vres+dVs).  (13)

Expression (13) yields a relational expression given by Expression (14)below for a capacity ratio k of the capacitance value C1 and thecapacitance value C2.

$\begin{matrix}{k > {\frac{dVs}{{{Vs}\; 1} - {Vres} + {dVs}} - 1}} & (14)\end{matrix}$

If the relationship of Expression (14) is satisfied, the amount ofelectric charge that is not discharged can be reduced. Accordingly,noise reduction may be achieved.

In this exemplary embodiment, the first voltage Vs1 is equal to 5 V, andthe reset voltage Vres is equal to 3.3 V. Since the second voltage Vs2is equal to 0 V, the amount of change dVs in the voltage on the firstelectrode 201 is −5 V. Thus, the value k is set to a value larger than0.52. Specifically, in this exemplary embodiment, the capacitance valueC1 of the first capacitor 103 is equal to 4 fF, and the capacitancevalue C2 of the second capacitor 111 is equal to 1 fF. That is, k=4 isobtained. This configuration may achieve more noise reduction.

In this exemplary embodiment, the area Sd of either the upper electrode211 or the lower electrode 213 of the first capacitor 103 in plan viewand the area Ss of the second electrode 209 in plan view satisfy arelationship of Sd>0.5×Ss. This configuration may make it easy to obtainthe relationship of the capacity ratio described above.

Further, as the value k increases, the noise reduction effect increases.Accordingly, in a case where the capacitance value C1 of the firstcapacitor 103 is equal to or greater than the capacitance value C2 ofthe second capacitor 111, the noise reduction effect may further beincreased.

The amount of change dVs in the voltage on the first electrode 201 isgiven by dVs=Vs2−Vs1 using the first voltage Vs1 and the second voltageVs2. Accordingly, Expression (14) is modified into Expression (15).

$\begin{matrix}{k > {\frac{{{Vs}\; 2} - {{Vs}\; 1}}{{{Vs}\; 2} - {Vres}} - 1}} & (15)\end{matrix}$

In particular, if the second voltage Vs2 is equal to 0 V, Expression(15) can be simplified into Expression (16).

$\begin{matrix}{k > {\frac{{Vs}\; 1}{Vres} - 1}} & (16)\end{matrix}$

An exemplary embodiment in which signal charges are electrons will nowbe described. If signal charges are electrons, the orientations of theinequality signs in Expression (8) and Expression (9) are changed.Accordingly, the orientations of the inequality signs in Expression (10)to Expression (11) are also changed. If signal charges are electrons,the reset voltage Vres is lower than the second voltage Vs2. Thus, thevalue given by Vs1−Vres+dVs=Vs2−Vres in Expression (11) is a positivevalue. That is, the relationship of (Vs1−Vres+dVs)>0 holds true. Thus,dividing both sides of Expression (11) by (Vs1−Vres+dVs) does not changethe orientation of the inequality sign. Consequently, as in the casewhere signal charges are holes, Expression (14) and Expression (15) areobtained.

The left-hand side of Expression (15) can be replaced with C1/C2 usingExpression (5). Since (Vs2−Vres)/(Vs2−Vres)=1, bringing the right-handside of Expression (15) to the common denominator yields Expression (17)as follows:

$\begin{matrix}{\frac{C\; 1}{C\; 2} > \frac{{Vres} - {{Vs}\; 1}}{{{Vs}\; 2} - {Vres}}} & (17)\end{matrix}$

Here, the relationship given by Expression (17) will be described. Thereset voltage Vres is an intermediate value between the first voltageVs1 and the second voltage Vs2.

As the reset voltage Vres approaches the first voltage Vs1, the value onthe right-hand side decreases. That is, even if the capacitance value C1of the first capacitor 103 is small, the gradient of the potential ofthe photoelectric conversion layer 205 can be inverted. If thedifference between the reset voltage Vres and the first voltage Vs1 issmall, the amount of electric charge that can be accumulated in thephotoelectric conversion layer 205 is small.

In contrast, as the reset voltage Vres approaches the second voltageVs2, the value on the right-hand side increases. That is, a large valueis used for the capacitance value C1 of the first capacitor 103. In thiscase, the difference between the reset voltage Vres and the firstvoltage Vs1 is large. Thus, the amount of electric charge that can beaccumulated in the photoelectric conversion layer 205 can be increased.

In terms of the balance between the saturated amount of electric chargeand the capacitance value C1 of the first capacitor 103, it ispreferable that the reset voltage Vres be in a range of 20% to 80% ofthe range with an upper limit and a lower limit (or a lower limit and anupper limit) equal to the first voltage Vs1 and the second voltage Vs2,respectively. For example, if the first voltage Vs1 is equal to 5 V andthe second voltage Vs2 is equal to 0 V, it is desirable that the resetvoltage Vres be in a range of 1 V to 4 V.

In a case where the photoelectric conversion device is used as an imagesensor of a camera or the like, a low power supply voltage is used for areduction in power consumption. For example, the power supply voltagesupplied to an image sensor is typically less than or equal to 5 V.Accordingly, values less than or equal to 5 V are also used for thevoltages in Expression (14) to Expression (17). In this case, thecapacitance value C1 of the first capacitor 103 and the capacitancevalue C2 of the second capacitor 111 satisfy the relationship describedabove, enabling noise reduction with low-voltage driving of thephotoelectric conversion device.

As described above, noise reduction may be achieved using therelationship between the capacitance value C1 of the first capacitor 103and the capacitance value C2 of the second capacitor 111 included in thephotoelectric conversion unit 101.

The numerical values given in the foregoing are merely examples, and arenot intended to be limiting.

Defect energy levels and the like may be present at the interfacebetween the photoelectric conversion layer 205 and the insulating layer207. In this case, it may be sufficient to take into account theflatband voltage by using a known technique.

Next, a method for driving the photoelectric conversion device accordingto this exemplary embodiment will be described. FIG. 7 illustrates atiming chart of drive signals used in the photoelectric conversiondevice according to this exemplary embodiment. In FIG. 7, drive signalsfor signal readout for the n-th row and the (n+1)-th row, or two rows intotal, are illustrated.

The drive signal pSEL is supplied to a gate of the selection transistor105. The drive signal pRES is supplied to a gate of the reset transistor102. The voltage signal Vs is supplied to the first electrode 201 of thephotoelectric conversion unit 101. The drive signal pTS is supplied tothe S/H switch 303. The drive signal pTN is supplied to the S/H switch305. The drive signal CSEL is supplied to the column driver circuit 150.

When the drive signal pSEL, the drive signal pRES, the drive signal pTN,or the drive signal pTS is in high level, the corresponding transistoror switch is turned on. When the drive signal pSEL, the drive signalpRES, the drive signal pTN, or the drive signal pTS is in low level, thecorresponding transistor or switch is turned off. The voltage signal Vsincludes the first voltage Vs1 and the second voltage Vs2.

The photoelectric conversion device according to this exemplaryembodiment performs a “rolling shutter” operation. Before time t1, thephotoelectric conversion units 101 of the pixels 100 in the n-th row andthe photoelectric conversion units 101 of the pixels 100 in the (n+1)-throw accumulate signal charge. Before time t1, furthermore, the voltagesignal Vs(n) for the n-th row and the voltage signal Vs(n+1) for the(n+1)-th row are each equal to the first voltage Vs1.

At time t1, the drive signal pSEL(n) rises to a high level, and theselection transistors 105 of the pixels 100 in the n-th row are turnedon. Accordingly, the amplifier transistors 104 of the pixels 100 in then-th row output a signal.

At time t1, the drive signal pRES(n) rises to a high level, and thereset transistors 102 of the pixels 100 in the n-th row are turned on.Accordingly, the voltage on the nodes B of the pixels 100 in the n-throw is reset to the reset voltage Vres. After that, at time t2, thedrive signal pRES(n) falls to a low level, and the reset transistors 102are turned off. The state of the energy band of each of thephotoelectric conversion units 101 at this time is illustrated in FIG.6A.

Then, the drive signal pTN(n) rises to a high level at time t3, andfalls to a low level at time t4. Accordingly, a noise signal includingreset noise (kTC1 illustrated in FIG. 6A) is held in the capacitors CTNof the column circuits 140.

At time t5, the voltage signal Vs(n) transitions from the first voltageVs1 to the second voltage Vs2. The state of the energy band of each ofthe photoelectric conversion units 101 at this time is illustrated inFIG. 6B. Subsequently, at time t6, the voltage signal Vs(n) transitionsfrom the second voltage Vs2 to the first voltage Vs1. The state of theenergy band of each of the photoelectric conversion units 101 at thistime is illustrated in FIG. 6C. Through the operation from time t5 totime t6, the signal charge is transferred in the way described above.Accordingly, the voltage Vp corresponding to the amount of accumulatedsignal charge is generated in the nodes B.

The drive signal pTS(n) rises to a high level at time t7, and falls to alow level at time t8. Accordingly, an optical signal including thevoltage Vp and the reset noise (kTC1 illustrated in FIG. 6C) is held inthe capacitors CTS of the column circuits 140.

Subsequently, the drive signal pRES(n) rises to a high level at time t9,and falls to a low level at time t10. Accordingly, the voltage on thenodes B of the pixels 100 in the n-th row is again reset to the resetvoltage Vres. The state of the energy band of each of the photoelectricconversion units 101 at this time is illustrated in FIG. 6D.

Thereafter, the pixels 100 in the n-th row start accumulation of signalcharge for the subsequent frame. The state of the energy band of each ofthe photoelectric conversion units 101 during the accumulation of signalcharge is illustrated in FIGS. 6E and 6F.

At time t11, the drive signal pSEL(n) falls to a low level, and thereadout of signals to the column circuits 140 from the pixels 100 in then-th row is completed.

The noise signals and the optical signals read out to the columncircuits 140 are output to the output unit 170 on a column-by-columnbasis in accordance with the drive signal CSEL. The output unit 170outputs differences between the optical signals and the noise signals tothe analog-to-digital conversion unit 180.

At time t12, the drive signal pSEL(n+1) rises to a high level, and theselection transistors 105 of the pixels 100 in the (n+1)-th row areturned on. In the subsequent operation, signals are read out from thepixels 100 in the (n+1)-th row. This operation is similar to theoperation from time t1 to time t11, and is not described herein.

In the photoelectric conversion device according to this exemplaryembodiment, as described above, the photoelectric conversion layer 205includes the quantum dots 10. This configuration enables a reduction inthe defect energy levels in the photoelectric conversion layer for thefollowing reasons. Because the size of the quantum dots is small, thelattice defects in the quantum dots can be reduced. Consequently, noisereduction may be achieved.

Second Exemplary Embodiment

Another exemplary embodiment will be described. This exemplaryembodiment is different from the first exemplary embodiment in a node towhich a voltage supply unit supplies a voltage. Thus, only portionsdifferent from the first exemplary embodiment are described. Portionsthat are substantially the same as those in the first exemplaryembodiment are not described.

FIG. 8 schematically illustrates a configuration of a pixel 100 of aphotoelectric conversion device according to this exemplary embodiment.Portions having substantially the same functions as those in FIG. 1A areassigned the same numerals. Equivalent circuits of the photoelectricconversion unit 101 according to this exemplary embodiment are the sameas those in the first exemplary embodiment. That is, FIGS. 1B and 1Cillustrate example equivalent circuits of the photoelectric conversionunit 101 according to this exemplary embodiment.

In this exemplary embodiment, a voltage Vd from a voltage supply unit410 is supplied to the second terminal of the first capacitor 103. Thevoltage supply unit 410 supplies at least a first voltage Vd1 and asecond voltage Vd2 different from the first voltage Vd1 to the secondterminal of the first capacitor 103.

If signal charges are holes, the second voltage Vd2 is a voltage higherthan the first voltage Vd1. If signal charges are holes, for example,the first voltage Vd1 is equal to 0 V, and the second voltage Vd2 isequal to 5 V. If signal charges are electrons, the second voltage Vd2 isa voltage lower than the first voltage Vd1. If signal charges areelectrons, for example, the first voltage Vd1 is equal to 5 V, and thesecond voltage Vd2 is equal to 0 V.

On the other hand, a predetermined voltage Vs is supplied to the firstelectrode 201 of the photoelectric conversion unit 101. In thisexemplary embodiment, a voltage of 3 V is supplied to the firstelectrode 201 of the photoelectric conversion unit 101. In FIG. 8, thefirst electrode 201 is included in the node A.

Next, the reset voltage Vres supplied by the reset transistor 102 willbe described. If signal charges are holes, the reset voltage Vres is avoltage lower than the voltage Vs supplied to the first electrode 201 ofthe photoelectric conversion unit 101. If signal charges are electrons,the reset voltage Vres is a voltage higher than the voltage Vs suppliedto the first electrode 201 of the photoelectric conversion unit 101.

In this exemplary embodiment, the voltage Vd on the node C is controlledto control the voltage on the node B coupled to the node C via the firstcapacitor 103. Thus, a direct current relationship in magnitude betweenthe voltage Vd supplied to the node C and the reset voltage Vres or thevoltage Vs supplied to the node A is not particularly limited.

FIG. 9 is a schematic diagram of an overall circuit configuration of thephotoelectric conversion device according to this exemplary embodiment.Portions having substantially the same functions as those in FIG. 2 areassigned the same numerals.

FIG. 9 schematically illustrates the planar structure of the firstelectrode 201 of the photoelectric conversion unit 101. The firstelectrode 201 is included in the node A illustrated in FIG. 8. Asillustrated in FIG. 9, photoelectric conversion units 101 of a pluralityof pixels 100 in a plurality of rows and a plurality of columns areconfigured to include a common first electrode 201. The voltage Vs issupplied to the first electrode 201.

In this exemplary embodiment, the voltage Vd to be supplied to thesecond terminals (the nodes C) of the first capacitors 103 is controlledindependently on a row-by-row basis. Thus, the row driver circuit 120selects a row to which the voltage Vd is supplied from the voltagesupply unit 410. Signs indicating rows, such as (n) and (n+1), areassigned to distinguish voltages Vd to be supplied to different rows. Inthis exemplary embodiment, the configuration described above enables aplurality of pixels 100 to be driven row-by-row.

Column circuits 140 according to this exemplary embodiment havesubstantially the same configuration as those of the first exemplaryembodiment. That is, FIG. 3 illustrates an equivalent circuit of thecolumn circuits 140 according to this exemplary embodiment. Also,similarly to the first exemplary embodiment, each of the column circuits140 may be an analog-to-digital conversion circuit. In this case, theanalog-to-digital conversion circuit includes a holding unit that holdsa digital signal, such as a memory or a counter. The holding unit holdsdigital signals into which a noise signal and an optical signal areconverted.

Next, the planar structure and cross-sectional structure of thephotoelectric conversion device according to this exemplary embodimentwill be described. FIG. 10 schematically illustrates the planarstructure of the photoelectric conversion device. FIG. 11 schematicallyillustrates the cross-sectional structure of the photoelectricconversion device. The cross section illustrated in FIG. 11 correspondsto the cross section taken along the line XI-XI in FIG. 10. In FIGS. 10and 11, portions which are substantially the same as those in FIGS. 4,5A, and 5B are assigned the same numerals.

As illustrated in FIG. 10 and FIG. 11, the lower electrodes 213 of thefirst capacitors 103 are connected to conductive members 420. Theconductive members 420 form wiring lines through which the voltage Vdfrom the voltage supply unit 410 is supplied. In this exemplaryembodiment, the conductive members 420 are disposed for the respectiverows, and the conductive member 420 for a certain row is electricallyisolated from the conductive members 420 for the other rows. Thisconfiguration enables the voltage Vd on the second terminals (the nodesC) of the first capacitors 103 to be controlled independently on arow-by-row basis.

The structure other than that described above is similar to that of thefirst exemplary embodiment, and is not described herein.

Next, the operation of each of the photoelectric conversion units 101according to this exemplary embodiment will be described. FIGS. 12A to12F schematically illustrate energy bands in each of the photoelectricconversion units 101. In FIGS. 12A to 12F, the energy bands of the firstelectrode 201, the blocking layer 203, the photoelectric conversionlayer 205, the insulating layer 207, and the second electrode 209 areillustrated. The vertical axis in FIGS. 12A to 12F represents thepotential of electrons. The potential of electrons increases along thevertical axis in an upward direction in FIGS. 12A to 12F. Accordingly,the voltage decreases along the vertical axis in an upward direction inFIGS. 12A to 12F. For the first electrode 201 and the second electrode209, the energy level of free electrons is illustrated. For the blockinglayer 203 and the photoelectric conversion layer 205, a band gap betweenthe energy level of the conduction band and the energy level of thevalence band is illustrated. The potential of the photoelectricconversion layer 205 at the interface between the photoelectricconversion layer 205 and the insulating layer 207 is referred to as the“surface potential of the photoelectric conversion layer 205” or simplyas the “surface potential”, for convenience.

In the operation of the photoelectric conversion unit 101, the followingsteps (1) to (6) are repeatedly performed: (1) the reset of the inputnode of the amplification unit, (2) the readout of a noise signal, (3)the discharge of signal charge from the photoelectric conversion unit,(4) the readout of an optical signal, (5) the reset before accumulationof signal charge is started, and (6) the accumulation of signal charge.In the following, the respective steps will be described.

FIG. 12A illustrates the state of the photoelectric conversion unit 101in step (1) to step (2). The voltage Vs is supplied to the firstelectrode 201. The voltage Vs is equal to, for example, 3 V. In thephotoelectric conversion layer 205, holes plotted using empty circlesare accumulated as signal charges generated during an exposure period.The surface potential of the photoelectric conversion layer 205 changesin the direction in which the surface potential decreases (i.e., in thedirection in which voltage increases) in accordance with the number ofaccumulated holes. In the case of accumulation of electrons, the surfacepotential changes in the direction in which the surface potentialincreases (i.e., in the direction in which voltage decreases) inaccordance with the number of accumulated electrons. In addition, thefirst voltage Vd1 is supplied to the node C. The first voltage Vd1 isequal to, for example, 0 V.

In this state, the reset transistor 102 is turned on. Accordingly, thevoltage on a node including the second electrode 209, that is, thevoltage on the node B illustrated in FIG. 8, is reset to the resetvoltage Vres. In this exemplary embodiment, the node B includes the gateof the amplifier transistor 104. Thus, the voltage at the gate of theamplifier transistor 104 is reset. The reset voltage Vres is equal to,for example, 1 V.

After that, the reset transistor 102 is turned off. Accordingly, thenode B is brought into an electrically floating state. In this case,reset noise (noise kTC1 illustrated in FIG. 12A) may be generated by thereset transistor 102.

The surface potential of the photoelectric conversion layer 205 maychange in accordance with a change in the voltage on the secondelectrode 209 during the reset operation. In this case, the direction inwhich the voltage on the second electrode 209 changes is opposite to thedirection in which the voltage on the second electrode 209 has changeddue to the accumulation of signal charge. For this reason, the holes ofthe signal charge remain accumulated in the photoelectric conversionlayer 205. In addition, the blocking layer 203 prevents injection ofholes from the first electrode 201. Thus, the amount of signal chargeaccumulated in the photoelectric conversion layer 205 does not change.

If the selection transistor 105 is in an on state, the amplifiertransistor 104 outputs a noise signal (Vres+kTC1) including the resetnoise from the pixel 100. The noise signal is held in the capacitor CTNof the column circuit 140.

FIGS. 12B and 12C illustrate the state of the photoelectric conversionunit 101 in step (3). First, the second voltage Vd2 is supplied to thenode C. Since holes are used as signal charges, the second voltage Vd2is a voltage higher than the first voltage Vs1. The second voltage Vd2is equal to, for example, 5 V.

In this case, the voltage on the second electrode 209 (the node B)changes in the same direction as the direction in which the voltage onthe node C changes. An amount of change dVB in the voltage on the secondelectrode 209 is determined in accordance with the ratio of thecapacitance value C1 of the first capacitor 103 connected to the secondelectrode 209 to the capacitance value C2 of the second capacitor 111included in the photoelectric conversion unit 101. The amount of changedVB in the voltage on the second electrode 209 with respect to an amountof change dVd in the voltage on the node C is given bydVB=dVd×C1/(C1+C2). The node B including the second electrode 209 mayalso include other capacitance components. The other capacitancecomponents have a much smaller capacitance value than the capacitancevalue C1 of the first capacitor 103. Thus, the capacitance value of thenode B can be regarded as being equal to the capacitance value C1 of thefirst capacitor 103.

In this exemplary embodiment, the amount of change dVB in the voltage onthe second electrode 209 is much larger than the difference (Vs−Vres)between the voltage Vs on the first electrode 201 and the reset voltageVres. Thus, the potential of the second electrode 209 is lower than thepotential of the first electrode 201, and the gradient of the potentialof the photoelectric conversion layer 205 is inverted. Accordingly, anelectron plotted using a solid circle is injected into the photoelectricconversion layer 205 from the first electrode 201. In addition, some orall of the holes accumulated in the photoelectric conversion layer 205as signal charges move to the blocking layer 203. The holes that havemoved are recombined with the majority carriers in the blocking layer203 and disappear. Consequently, the holes in the photoelectricconversion layer 205 are discharged from the photoelectric conversionlayer 205. For the depletion of the entire photoelectric conversionlayer 205, all the holes accumulated as signal charges are discharged.

Then, in the state illustrated in FIG. 12C, the first voltage Vd1 issupplied to the node C. Accordingly, the gradient of the potential ofthe photoelectric conversion layer 205 is inverted again. Thus, theelectrons injected into the photoelectric conversion layer 205 in thestate illustrated in FIG. 12B are discharged from the photoelectricconversion layer 205. On the other hand, the blocking layer 203 preventsinjection of holes into the photoelectric conversion layer 205 from thefirst electrode 201. Accordingly, the surface potential of thephotoelectric conversion layer 205 changes in accordance with the numberof holes that have been accumulated. In accordance with the change insurface potential, the voltage on the second electrode 209 changes fromthe reset state by a voltage Vp corresponding to the number of holesthat have disappeared. That is, the voltage Vp corresponding to thenumber of holes accumulated as signal charges appears at the node B. Thevoltage Vp corresponding to the number of accumulated holes is referredto as an “optical signal component”.

In the state illustrated in FIG. 12C, the selection transistor 105 isturned on. Accordingly, the amplifier transistor 104 outputs an opticalsignal (Vp+Vres+kTC1) from the pixel 100. The optical signal is held inthe capacitor CTS of the column circuit 140. The difference between thenoise signal (Vres+kTC1) read out in step (2) and the optical signal(Vp+Vres+kTC1) read out in step (4) is a signal based on the voltage Vpcorresponding to the accumulated signal charge.

FIG. 12D illustrates the state of the photoelectric conversion unit 101in step (5). The reset transistor 102 is turned on, and the voltage onthe node B is reset to the reset voltage Vres. After that, the resettransistor 102 is turned off. In the way described above, the node B isreset before the accumulation of signal charge is started, enabling theoptical signal component for the preceding frame which has beenaccumulated in the node B to be removed. Accordingly, the dynamic rangemay be prevented from being narrowed in accordance with the accumulationof optical signals in the node B. Note that the reset beforeaccumulation of signal charge is started in step (5) may not necessarilybe performed.

Also in this case, reset noise (noise kTC2 illustrated in FIG. 12D) maybe generated by the reset transistor 102. The generated reset noise canbe removed through the reset operation in step (1) after the completionof the accumulation period.

FIGS. 12E and 12F illustrate the state of the photoelectric conversionunit 101 in step (6). The voltage Vs is supplied to the first electrode201, and the reset voltage Vres is supplied to the node B. The resetvoltage Vres is lower than the voltage Vs on the first electrode 201.Thus, the electrons in the photoelectric conversion layer 205 aredischarged to the first electrode 201. In contrast, the holes in thephotoelectric conversion layer 205 move to the interface between thephotoelectric conversion layer 205 and the insulating layer 207.However, the holes are not movable to the insulating layer 207, and arethus accumulated in the photoelectric conversion layer 205. In addition,as described previously, the blocking layer 203 prevents the holes frombeing injected into the photoelectric conversion layer 205. In thisstate, when light enters the photoelectric conversion layer 205, onlythe holes in the electron-hole pairs generated by photoelectricconversion are accumulated in the photoelectric conversion layer 205 assignal charges. After the accumulation operation has been performed fora certain period, the operations in steps (1) to (6) are repeatedlyperformed.

The accumulated holes cause a change in the surface potential of thephotoelectric conversion layer 205. In accordance with the change insurface potential, the voltage on the second electrode 209 increases.This increase is represented by Vp0 in FIG. 12F. In the reset operationin FIG. 12A, as described above, the voltage on the second electrode 209changes so as to cancel out the change of the voltage Vp0. That is, thevoltage on the second electrode 209 decreases. Accordingly, the surfacepotential of the photoelectric conversion layer 205 changes in thedirection in which the surface potential increases.

If signal charges are electrons, the second voltage Vd2 is a voltagelower than the first voltage Vd1. Thus, the gradient of the potentialillustrated in FIGS. 12A to 12F is inverted. The other operations aresubstantially the same.

In the operation described with reference to FIGS. 12A to 12F, thegradient of the potential of the photoelectric conversion layer 205 isinverted in the state illustrated in FIG. 12B, enabling the discharge ofthe accumulated holes. Non-inversion of the gradient of the potential ofthe photoelectric conversion layer 205 causes the occurrence of electriccharge that is not discharged. Thus, noise may occur. Here, as theamount by which the amount of change dVB in the voltage on the secondelectrode 209 (the node B) is larger than the difference (Vs−Vres)between the voltage Vs on the first electrode 201 and the reset voltageVres increases, the potential gradient is more likely to be inverted.That is, as the amount by which the amount of change dVB in the voltageon the second electrode 209 is larger than the difference (Vs−Vres)between the voltage Vs on the first electrode 201 and the reset voltageVres increases, further noise reduction is achievable.

As described above, there is a relationship represented bydVB=dVd×C1/(C1+C2) between the amount of change dVd in the voltage onthe node C and the amount of change dVB in the voltage on the node B.That is, as the capacitance value C1 of the node B increases, the amountof change dVB in the voltage on the node B increases.

In this exemplary embodiment, the first capacitor 103 is connected tothe second electrode 209. Thus, the capacitance value C1 of the node Bcan be increased. This configuration enables an increase in the amountof change dVB in the voltage on the node B. Consequently, depletion ofthe photoelectric conversion layer 205 is more likely to be achieved,resulting in a reduction in electric charge that is not discharged.According to this exemplary embodiment, therefore, noise reduction maybe achieved.

A description will be given of a comparative example in which the firstcapacitor 103 is not connected to the node B. In this configuration, thenode B has a capacitance that may include a capacitance component due toa PN junction in a semiconductor region and a parasitic capacitancecomponent of a wiring line. Since the capacitance components describedabove are negligibly smaller than the capacitance value C2 of the secondcapacitor 111 included in the photoelectric conversion unit 101, thevalue given by C1/(C1+C2) is substantially equal to zero. Thus, even ifthe voltage Vd on the node C is changed, the voltage on the node B doesnot substantially change. In this case, the gradient of the potentialmay not be inverted in the state illustrated in FIG. 12B, resulting in apossibility that some of the holes accumulated as signal charges willnot be discharged. In this exemplary embodiment, by contrast to thecomparative example, the amount of signal charge that is not dischargedcan be reduced, resulting in noise reduction.

A description will now be given of the relationship among thecapacitance value C1 of the first capacitor 103, the capacitance valueC2 of the second capacitor 111 included in the photoelectric conversionunit 101, and the voltage supplied to each unit. In this exemplaryembodiment, the capacitance value C1 and the capacitance value C2 arerepresented by Expression (4) and Expression (1) in the first exemplaryembodiment, respectively. A detailed description is not given here.

In this exemplary embodiment, the voltage Vd on the nodes C iscontrolled using the first voltage Vd1 and the second voltage Vd2 toaccumulate signal charge and discharge the signal charge due to thedepletion of the photoelectric conversion layer 205. The capacitancevalue C1 of the first capacitor 103 and the capacitance value C2 of thesecond capacitor 111 satisfy the following relationship, therebyachieving a reduction in the electric charge remaining in thephotoelectric conversion layer 205 during the discharge of signal chargedescribed above. An exemplary embodiment in which signal charges areholes will be described first.

In the following, for simplicity, the capacitance value C1 of the firstcapacitor 103 is k times as large as the capacitance value C2 of thesecond capacitor 111. That is, the capacitance value C1 and thecapacitance value C2 have a relationship of Expression (18) as follows:

C1=k×C2.  (18)

As described previously, the amount of change dVd in the voltage on thenode C and the amount of change dVB in the voltage on the secondelectrode 209 (the node B) have a relationship given by Expression (19)as follows:

dVB=dVd×C1/(C1+C2).  (19)

Expression (18) and Expression (19) yield Expression (20) as follows:

dVB=dVd×k/(1+k).  (20)

To accumulate holes as signal charges, it is desirable that the voltageVs supplied to the first electrode 201 (the node A) and the resetvoltage Vres satisfy a relationship of Expression (21) as follows:

Vs>Vres.  (21)

To transfer the holes of the signal charge, it is desirable that thevoltage Vs on the first electrode 201 (the node A), the reset voltageVres, and the amount of change dVB in the voltage on the secondelectrode 209 satisfy a relationship of Expression (22) as follows:

Vs<Vres+dVB.  (22)

If the relationship of Expression (21) is satisfied, the gradient of thepotential that allows holes to drift toward the insulating layer 207 canbe formed in the photoelectric conversion layer 205. If the relationshipof Expression (22) is satisfied, it is easy to reverse the gradient ofthe potential of the photoelectric conversion layer 205.

Expression (20) and Expression (22) yield Expression (23).

Vs−Vres<dVd×k/(1+k)  (23)

In an exemplary embodiment in which signal charges are holes, the secondvoltage Vd2 is higher than the first voltage Vd1. That is, the amount ofchange dVd (=Vd2−Vd1) in the voltage on the node C has a positive value.Accordingly, dividing both sides of Expression (23) by dVd does notchange the orientation of the inequality sign.

Accordingly, Expression (23) yields a relational expression given byExpression (24) below for a capacity ratio k of the capacitance value C1and the capacitance value C2.

$\begin{matrix}{{1 - \frac{1}{1 + k}} > \frac{{Vs} - {Vres}}{dVd}} & (24)\end{matrix}$

If the relationship of Expression (24) is satisfied, the amount ofelectric charge that is not discharged can be reduced. Accordingly,noise reduction may be achieved.

In this exemplary embodiment, the voltage Vs on the first electrode 201is equal to 3 V, and the reset voltage Vres is equal to 1 V. Since thefirst voltage Vd1 is equal to 0 V and the second voltage Vs2 is equal to5 V, the amount of change dVd in the voltage on the node C is equal to 5V. Thus, k>⅔ is set. Specifically, in this exemplary embodiment, thecapacitance value C1 of the first capacitor 103 is equal to 4 fF, andthe capacitance value C2 of the second capacitor 111 is equal to 1 fF.That is, k=4 is obtained. This configuration may achieve more noisereduction.

In this exemplary embodiment, the area Sd of either the upper electrode211 or the lower electrode 213 of the first capacitor 103 in plan viewand the area Ss of the second electrode 209 in plan view satisfy arelationship of Sd>0.5×Ss. This configuration may make it easy to obtainthe relationship of the capacity ratio described above.

Further, as the value k increases, the noise reduction effect increases.Accordingly, in a case where the capacitance value C1 of the firstcapacitor 103 is equal to or greater than the capacitance value C2 ofthe second capacitor 111, the noise reduction effect may further beincreased.

The amount of change dVd in the voltage on the node C is given bydVd=Vd2−Vd1 using the first voltage Vd1 and the second voltage Vd2.Further, the left-hand side of Expression (24) can be rewritten asC1/(C1+C2) using Expression (18). Accordingly, Expression (24) ismodified into Expression (25).

$\begin{matrix}{\frac{C\; 1}{{C\; 1} + {C\; 2}} > \frac{{Vs} - {Vres}}{{{Vd}\; 2} - {{Vd}\; 1}}} & (25)\end{matrix}$

An exemplary embodiment in which signal charges are electrons will nowbe described. If signal charges are electrons, the orientations of theinequality signs in Expression (21) and Expression (22) are changed.Accordingly, the orientation of the inequality sign in Expression (23)is also changed. That is, if signal charges are electrons, Expression(26) below is obtained.

Vs−Vres>dVd×k/(1+k)  (26)

Note that, in an exemplary embodiment in which signal charges areelectrons, the second voltage Vd2 is lower than the first voltage Vd1.That is, the amount of change dVd (=Vd2−Vd1) in the voltage on the nodeC has a negative value. Accordingly, dividing both sides of Expression(26) by dVd changes the orientation of the inequality sign.Consequently, Expression (24) and Expression (25) are obtained as in thecase where signal charges are holes.

Here, the relationship given by Expression (25) will be described. Asthe reset voltage Vres approaches the voltage Vs supplied to the firstelectrode 201 of the photoelectric conversion unit 101, the value on theright-hand side decreases. That is, even if the capacitance value C1 ofthe first capacitor 103 is small, the gradient of the potential of thephotoelectric conversion layer 205 can be inverted. If the differencebetween the reset voltage Vres and the voltage Vs supplied to the firstelectrode 201 is small, the amount of electric charge that can beaccumulated in the photoelectric conversion layer 205 is small.

In contrast, as the difference between the reset voltage Vres and thevoltage Vs increases, the value on the right-hand side increases. Thatis, a large value is used for the capacitance value C1 of the firstcapacitor 103. In this case, the difference between the reset voltageVres and the first voltage Vs1 is large. Thus, the amount of electriccharge that can be accumulated in the photoelectric conversion layer 205can be increased.

In terms of the balance between the saturated amount of electric chargeand the capacitance value C1 of the first capacitor 103, it ispreferable that the difference between the reset voltage Vres and thevoltage Vs be in a range of 20% to 80% of the difference between thefirst voltage Vs1 and the second voltage Vs2. For example, if the firstvoltage Vs1 is equal to 0 V and the second voltage Vs2 is equal to 5 V,it is desirable that the difference from the reset voltage Vres be in arange of 1 V to 4 V.

In particular, an increase in the difference between the first voltageVd1 and the second voltage Vd2 may reduce the capacitance value C1 ofthe first capacitor 103 even if the difference between the reset voltageVres and the voltage Vs is large. In a case where the photoelectricconversion device is used as an image sensor of a camera or the like,however, a low power supply voltage is used for a reduction in powerconsumption. For example, the power supply voltage supplied to an imagesensor is typically less than or equal to 5 V. Accordingly, values lessthan or equal to 5 V are also used for the voltages in Expression (24)to Expression (25). It is thus difficult to increase the differencebetween the first voltage Vd1 and the second voltage Vd2. In this case,the capacitance value C1 of the first capacitor 103 and the capacitancevalue C2 of the second capacitor 111 satisfy the relationship describedabove, enabling noise reduction with low-voltage driving of thephotoelectric conversion device.

As described above, noise reduction may be achieved using therelationship between the capacitance value C1 of the first capacitor 103and the capacitance value C2 of the second capacitor 111 included in thephotoelectric conversion unit 101.

The numerical values given in the foregoing are merely examples, and arenot intended to be limiting. Defect energy levels and the like may bepresent at the interface between the photoelectric conversion layer 205and the insulating layer 207. In this case, it may be sufficient to takeinto account the flatband voltage by using a known technique.

Next, a method for driving the photoelectric conversion device accordingto this exemplary embodiment will be described. FIG. 13 illustrates atiming chart of drive signals used in the photoelectric conversiondevice according to this exemplary embodiment. In FIG. 13, drive signalsfor signal readout for the n-th row and the (n+1)-th row, or two rows intotal, are illustrated.

The difference from the driving method according to the first exemplaryembodiment is that a voltage signal Vd is supplied to the nodes Cillustrated in FIG. 8. In FIG. 13, the timing chart of the voltagesignal Vd is illustrated. The voltage signal Vd includes the firstvoltage Vd1 and the second voltage Vd2. The period during which thevoltage signal Vs is the first voltage Vs1 in the first exemplaryembodiment corresponds to a period during which the voltage signal Vd isthe first voltage Vd1 in this exemplary embodiment. The period duringwhich the voltage signal Vs is the second voltage Vs2 in the firstexemplary embodiment corresponds to a period during which the voltagesignal Vd is the second voltage Vd2 in this exemplary embodiment.

The timing chart of the other drive signals is substantially the same asthat in FIG. 7. Thus, a detailed description is not given here.

In the photoelectric conversion device according to this exemplaryembodiment, as described above, the photoelectric conversion layer 205includes the quantum dots 10. This configuration enables a reduction inthe defect energy levels in the photoelectric conversion layer for thefollowing reasons. Because the size of the quantum dots is small, thelattice defects in the quantum dots can be reduced. Consequently, noisereduction may be achieved.

Third Exemplary Embodiment

Another exemplary embodiment will be described. This exemplaryembodiment is different from the first exemplary embodiment and thesecond exemplary embodiment in that a switch is disposed between thephotoelectric conversion unit and the input node of the amplificationunit. Thus, only portions different from the first exemplary embodimentor the second exemplary embodiment are described. Portions that aresubstantially the same as those in any of the first exemplary embodimentand the second exemplary embodiment are not described.

FIG. 14 schematically illustrates the configuration of pixels 100 of aphotoelectric conversion device according to this exemplary embodiment.In FIG. 14, four pixels 100 arranged in two rows and two columns areillustrated. Portions having substantially the same functions as thosein FIG. 1A are assigned the same numerals. Each of the photoelectricconversion units 101 has a structure similar to that in the firstexemplary embodiment. Thus, the cross-sectional structure of thephotoelectric conversion units 101 is not illustrated in FIG. 14.

In this exemplary embodiment, switches 501 are disposed in electricalpaths between the photoelectric conversion units 101 and the firstcapacitors 103. In other words, the first capacitors 103 areelectrically connected to the photoelectric conversion units 101 via theswitches 501. The switches 501 are also disposed in electrical pathsbetween the photoelectric conversion units 101 and the amplifiertransistors 104. In other words, the amplifier transistors 104 areelectrically connected to the photoelectric conversion units 101 via theswitches 501. The gates of the amplifier transistors 104 and the firstterminals of the first capacitors 103 are included in the nodes B.

The switches 501 control electrical conduction between the photoelectricconversion units 101 and the nodes B. Turning off both the switches 501and the reset transistors 102 brings the nodes B into an electricallyfloating state.

A drive signal pGS is supplied to the switches 501. Signs indicatingrows, such as (n) and (n+1), are assigned to distinguish drive signalspGS to be supplied to different rows.

The configuration of each of the pixels 100 according to this exemplaryembodiment is substantially the same as that in the first exemplaryembodiment, except that the switch 501 is disposed. Also, the overallconfiguration of the photoelectric conversion device according to thisexemplary embodiment is also substantially the same as that in the firstexemplary embodiment.

The configuration described above enables the exposure periods for allrows to coincide. A so-called global electronic shutter is achieved.Since the drive signal pGS is supplied independently on a row-by-rowbasis, the switching between a global electronic shutter operation modeand a rolling shutter operation mode is also achieved.

In this exemplary embodiment, as illustrated in FIG. 14, the voltage Vsfrom the voltage supply unit 110 is supplied to the nodes A to which thefirst terminals of the photoelectric conversion units 101 are connected.Similarly to the first exemplary embodiment, the voltage supply unit 110controls the voltage on the nodes A using at least the first voltage Vs1and the second voltage Vs2. This configuration enables the accumulationof electric charge in the photoelectric conversion units 101 and thedischarge or transfer of the electric charge from the photoelectricconversion units 101.

Next, a method for driving the photoelectric conversion device accordingto this exemplary embodiment will be described. FIG. 15 illustrates atiming chart of drive signals used in the photoelectric conversiondevice according to this exemplary embodiment. In FIG. 15, drive signalsfor signal readout for the n-th row and the (n+1)-th row, or two rows intotal, are illustrated.

The difference from the driving method according to the first exemplaryembodiment is that the drive signal pGS is supplied to the switch 501.In FIG. 15, the timing chart of the drive signal pGS is illustrated.When the drive signal pGS is in high level, the switch 501 is turned on.When the drive signal pGS is in low level, the switch 501 is turned off.

The photoelectric conversion device according to this exemplaryembodiment is configured to perform a global electronic shutteroperation. Before time t1, the photoelectric conversion units 101 of thepixels 100 in the n-th row and the photoelectric conversion units 101 ofthe pixels 100 in the (n+1)-th row accumulate signal charge. Before timet1, furthermore, the voltage signal Vs(n) for the n-th row and thevoltage signal Vs(n+1) for the (n+1)-th row are each equal to the firstvoltage Vs1.

At time t1, the drive signal pRES(n) and the drive signal pRES(n+1) riseto a high level, and the reset transistors 102 of the pixels 100 in then-th row are turned on. Accordingly, the voltage on the nodes B of thepixels 100 in the n-th row and the voltage on the nodes B of the pixels100 in the (n+1)-th row are each reset to the reset voltage Vres. Afterthat, at time t2, the drive signal pRES(n) and the drive signalpRES(n+1) fall to a low level, and the reset transistors 102 of thepixels 100 are turned off.

At time t3, the drive signal pGS(n) and the drive signal pGS(n+1) riseto a high level. Accordingly, the switches 501 are turned on. Thus, eachof the pixels 100 according to this exemplary embodiment has theequivalent circuit illustrated in FIG. 1B or 1C.

At time t4, the voltage signal Vs(n) and the voltage signal Vs(n+1)transition from the first voltage Vs1 to the second voltage Vs2.Subsequently, at time t5, the voltage signal Vs(n) and the voltagesignal Vs(n+1) transition from the second voltage Vs2 to the firstvoltage Vs1. Through the operation from time t4 to time t5, the signalcharge is transferred. Accordingly, the voltage Vp corresponding to theamount of accumulated signal charge is generated in the nodes B. Theoperation at this time is substantially the same as that described inthe first exemplary embodiment with reference to FIGS. 6A to 6F. Thatis, the state of the energy band of each of the photoelectric conversionunits 101 at this time is illustrated in FIG. 6B and FIG. 6C.

At time t6, the drive signal pGS(n) and the drive signal pGS(n+1) fallto a low level. Accordingly, the switches 501 are turned off. Thisresults in the disconnection of electrical conduction between thephotoelectric conversion units 101 and the nodes B. Thus, while thenodes B hold the voltage Vp corresponding to the amount of signal chargefor the preceding frame, the photoelectric conversion units 101 canaccumulate signal charge for the subsequent frame. In this exemplaryembodiment, the pixels 100 in a plurality of rows are capable ofperforming the operation described above in parallel. That is, thephotoelectric conversion units 101 of the pixels 100 in a plurality ofrows are capable of simultaneously starting the accumulation of signalcharge.

In the subsequent operation, optical signals are read out row-by-row. Attime t7, the drive signal pSEL(n) rises to a high level. At time t7, thedrive signal pTS(n) also rises to a high level. Accordingly, theamplifier transistors 104 of the pixels 100 in the n-th row output anoptical signal. The optical signals output from the pixels 100 in then-th row are held in the capacitors CTS. The optical signals held in thecapacitors CTS are output to the output unit 170 on a column-by-columnbasis after time t9.

At time t10, the readout of optical signals for the (n+1)-th row isstarted. This operation is similar to that for the n-th row, and is notdescribed herein.

Through the operation described above, signal readout based on theglobal electronic shutter operation is achieved. In FIG. 15, only thedrive signals for the n-th row and the (n+1)-th row are illustrated.Note that the operation from time t1 to time t6 may be performedsimultaneously for all rows.

In this exemplary embodiment, furthermore, the drive signals illustratedin FIG. 7 may be supplied while the drive signal pGS is kept at a highlevel. This enables signal readout based on the rolling shutteroperation in a way similar to that in the first exemplary embodiment.

Also in this exemplary embodiment, the photoelectric conversion layer205 includes the quantum dots 10. Thus, a noise reduction effect may beachieved.

Fourth Exemplary Embodiment

Another exemplary embodiment will be described. This exemplaryembodiment is different from the first exemplary embodiment and thesecond exemplary embodiment in that a switch is disposed between thephotoelectric conversion unit and the input node of the amplificationunit. The difference between this exemplary embodiment and the thirdexemplary embodiment is as follows: In the third exemplary embodiment,the voltage on the node A is controlled, whereas, in this exemplaryembodiment, the voltage on the node C is controlled. Thus, only portionsdifferent from the first exemplary embodiment to the third exemplaryembodiment are described. Portions that are substantially the same asthose in any of the first exemplary embodiment to the third exemplaryembodiment are not described.

FIG. 16 schematically illustrates the configuration of pixels 100 of aphotoelectric conversion device according to this exemplary embodiment.In FIG. 16, four pixels 100 arranged in two rows and two columns areillustrated. Portions having substantially the same functions as thosein FIG. 8 are assigned the same numerals. Each of the photoelectricconversion units 101 has a structure similar to that in the secondexemplary embodiment. Thus, the cross-sectional structure of thephotoelectric conversion units 101 is not illustrated in FIG. 16.

In this exemplary embodiment, switches 501 are disposed in electricalpaths between the photoelectric conversion units 101 and the firstcapacitors 103. In other words, the first capacitors 103 areelectrically connected to the photoelectric conversion units 101 via theswitches 501. The switches 501 are also disposed in electrical pathsbetween the photoelectric conversion units 101 and the amplifiertransistors 104. In other words, the amplifier transistors 104 areelectrically connected to the photoelectric conversion units 101 via theswitches 501. The gates of the amplifier transistors 104 and the firstterminals of the first capacitors 103 are included in the nodes B.

The switches 501 control electrical conduction between the photoelectricconversion units 101 and the nodes B. Turning off both the switches 501and the reset transistors 102 brings the nodes B into an electricallyfloating state.

A drive signal pGS is supplied to the switches 501. Signs indicatingrows, such as (n) and (n+1), are assigned to distinguish drive signalspGS to be supplied to different rows.

The configuration of each of the pixels 100 according to this exemplaryembodiment is substantially the same as that in the second exemplaryembodiment, except that the switch 501 is disposed. Also, the overallconfiguration of the photoelectric conversion device according to thisexemplary embodiment is also substantially the same as that in thesecond exemplary embodiment.

The configuration described above enables the exposure periods for allrows to coincide. A so-called global electronic shutter is achieved.Since the drive signal pGS is supplied independently on a row-by-rowbasis, the switching between a global electronic shutter operation modeand a rolling shutter operation mode is also achieved.

In this exemplary embodiment, as illustrated in FIG. 16, the voltage Vdfrom the voltage supply unit 410 is supplied to the nodes C coupled tothe nodes B via the first capacitors 103. Similarly to the secondexemplary embodiment, the voltage supply unit 410 controls the voltageon the nodes C using at least the first voltage Vd1 and the secondvoltage Vd2. This configuration enables the accumulation of electriccharge in the photoelectric conversion units 101 and the discharge ortransfer of the electric charge from the photoelectric conversion units101.

Next, a method for driving the photoelectric conversion device accordingto this exemplary embodiment will be described. FIG. 17 illustrates atiming chart of drive signals used in the photoelectric conversiondevice according to this exemplary embodiment. In FIG. 17, drive signalsfor signal readout for the n-th row and the (n+1)-th row, or two rows intotal, are illustrated.

The difference from the driving method according to the third exemplaryembodiment is that the voltage signal Vd is supplied to the nodes Cillustrated in FIG. 16. In FIG. 17, the timing chart of the voltagesignal Vd is illustrated. The voltage signal Vd includes the firstvoltage Vd1 and the second voltage Vd2. The period during which thevoltage signal Vs is the first voltage Vs1 in the third exemplaryembodiment corresponds to a period during which the voltage signal Vd isthe first voltage Vd1 in this exemplary embodiment. The period duringwhich the voltage signal Vs is the second voltage Vs2 in the thirdexemplary embodiment corresponds to a period during which the voltagesignal Vd is the second voltage Vd2 in this exemplary embodiment.

The timing chart of the other drive signals is substantially the same asthat in FIG. 15. Thus, a detailed description is not given here.

In this exemplary embodiment, in the manner described above, signalreadout based on the global electronic shutter operation is achieved. Inthis exemplary embodiment, furthermore, the drive signals illustrated inFIG. 7 may be supplied while the drive signal pGS is kept at a highlevel. This enables signal readout based on the rolling shutteroperation in a way similar to that in the first exemplary embodiment.

Also in this exemplary embodiment, the photoelectric conversion layer205 includes the quantum dots 10. Thus, a noise reduction effect may beachieved.

Fifth Exemplary Embodiment

Another exemplary embodiment will be described. This exemplaryembodiment is different from the first exemplary embodiment to thefourth exemplary embodiment in that each pixel includes a clamp circuitconnected downstream of the amplification unit. Thus, only portionsdifferent from the first exemplary embodiment to the fourth exemplaryembodiment are described. Portions that are substantially the same asthose in any of the first exemplary embodiment to the fourth exemplaryembodiment are not described.

FIG. 18 schematically illustrates the configuration of pixels 100 of aphotoelectric conversion device according to this exemplary embodiment.In FIG. 18, four pixels 100 arranged in two rows and two columns areillustrated. Portions having substantially the same functions as thosein FIG. 1A are assigned the same numerals. Each of the photoelectricconversion units 101 has a structure similar to that in any of the firstto fourth exemplary embodiments. Thus, the cross-sectional structure ofthe photoelectric conversion units 101 is not illustrated in FIG. 18.

In this exemplary embodiment, each of the pixels 100 includes twoamplification units. A first amplification unit is a source followercircuit including a first amplifier transistor 611 and a current source612. A second amplification unit includes a second amplifier transistor631. The second amplifier transistor 631 is connected to the output line130 via the selection transistor 105. The second amplifier transistor631 and the current source 160 connected to the output line 130constitute a source follower circuit.

Each of the pixels 100 further includes a clamp circuit for implementinga global electronic shutter. The clamp circuit includes a clamp switch621, a clamp capacitor 622, and a clamp voltage supply switch 623. Theclamp switch 621 is disposed in an electrical path between the node B towhich the first capacitor 103 is connected and an input node of thesecond amplification unit of the pixel 100. A drive signal pGS issupplied to the clamp switch 621. A drive signal pCL is supplied to theclamp voltage supply switch 623.

The clamp circuit clamps the noise signal output from the firstamplification unit. After that, the first amplification unit outputs anoptical signal. Thus, the clamp circuit is capable of removing noisesuch as reset noise included in the optical signal. This configurationenables implementation of a global electronic shutter operation whileremoving random noise such as reset noise.

Next, a method for driving the photoelectric conversion device accordingto this exemplary embodiment will be described. FIG. 19 illustrates atiming chart of drive signals used in the photoelectric conversiondevice according to this exemplary embodiment. In FIG. 19, drive signalsfor signal readout for the n-th row and the (n+1)-th row, or two rows intotal, are illustrated.

The difference from the driving method according to the first exemplaryembodiment is that the drive signal pGS is supplied to the switch 501.In FIG. 19, the timing chart of the voltage signal Vd is illustrated.When a drive signal is in high level, the corresponding switch is turnedon. When a drive signal is in low level, the corresponding switch isturned off.

At time t1, the drive signal pGS(n) and the drive signal pGS(n+1) riseto a high level. At time t2, the drive signal pRES(n) and the drivesignal pRES(n+1) rise to a high level. At time t2, furthermore, thedrive signal pCL(n) and the drive signal pCL(n+1) also rise to a highlevel. After that, at time t3, the drive signal pRES(n) and the drivesignal pRES(n+1) fall to a low level. At time t4, the drive signalpCL(n) and the drive signal pCL(n+1) fall to a low level. Accordingly,the clamp circuits of the pixels 100 in the n-th row and the (n+1)-throw clamp the noise signals.

Subsequently, in the period from time t5 to time t6, the voltage signalVd(n) and the voltage signal Vd(n+1) are the second voltage Vd2. Thus,the accumulated signal charge is transferred. Since the clamp switch 621is in an on state, a voltage Vp corresponding to the amount of signalcharge is generated in the clamp capacitor 622.

After that, at time t7, the drive signal pGS(n) and the drive signalpGS(n+1) fall to a low level. Accordingly, the clamp circuits of thepixels 100 are electrically separated from the photoelectric conversionunits 101.

In the subsequent operation, optical signals are read out row-by-row.This operation is substantially the same as that in the third exemplaryembodiment or the fourth exemplary embodiment, and is not describedherein.

Through the operation described above, a global electronic shutteroperation is implemented. In this exemplary embodiment, furthermore,each of the pixels 100 includes a clamp circuit. This configurationenables a reduction in random noise such as reset noise.

Sixth Exemplary Embodiment

Another exemplary embodiment will be described. This exemplaryembodiment is different from the first exemplary embodiment to the fifthexemplary embodiment in that each pixel includes a sample-and-holdcircuit connected downstream of the amplification unit. Thus, onlyportions different from the first exemplary embodiment to the fifthexemplary embodiment are described. Portions that are substantially thesame as those in any of the first exemplary embodiment to the fifthexemplary embodiment are not described.

FIG. 20 schematically illustrates the configuration of pixels 100 of aphotoelectric conversion device according to this exemplary embodiment.In FIG. 20, four pixels 100 arranged in two rows and two columns areillustrated. Portions having substantially the same functions as thosein FIG. 1A or FIG. 18 are assigned the same numerals. Each of thephotoelectric conversion units 101 has a structure similar to that inany of the first to fifth exemplary embodiments. Thus, thecross-sectional structure of the photoelectric conversion units 101 isnot illustrated in FIG. 20.

In this exemplary embodiment, each of the pixels 100 includes twoamplification units. A first amplification unit is a source followercircuit including the first amplifier transistor 611 and the currentsource 612. A second amplification unit includes the second amplifiertransistor 631. The second amplifier transistor 631 is connected to theoutput line 130 via the selection transistor 105. The second amplifiertransistor 631 and the current source 160 connected to the output line130 constitute a source follower circuit.

Each of the pixels 100 further includes a sample-and-hold circuit(hereinafter referred to as an “S/H circuit”) for implementing a globalelectronic shutter. The pixel 100 includes a noise signal S/H circuitand an optical signal S/H circuit. The noise signal S/H circuit holds anoise signal output from the first amplification unit. The opticalsignal S/H circuit holds an optical signal output from the firstamplification unit. The noise signal S/H circuit includes a capacitor701, a first switch 711, and a second switch 721. The optical signal S/Hcircuit includes a capacitor 702, a first switch 712, and a secondswitch 722.

This configuration enables implementation of a global electronic shutteroperation while removing random noise such as reset noise.

A driving method according to this exemplary embodiment will now bedescribed. A description will be given here of only the driving of theS/H circuits for performing a global electronic shutter operation.

First, the first switches 711 of the noise signal S/H circuits in thepixels 100 in all rows are turned on in the state where input nodes ofthe first amplification units are reset. Accordingly, noise signals areheld in the capacitors 701. Then, a signal charge transfer operation isperformed. This operation is similar to that in any of the firstexemplary embodiment to the fourth exemplary embodiment. Then, the firstswitches 712 of the optical signal S/H circuits in the pixels 100 in allrows are turned on. Accordingly, optical signals are held in thecapacitors 702. After that, the second switches 721 and 722 are turnedon row-by-row. Accordingly, signals from the pixels 100 are read outrow-by-row. The signals output from the pixels 100 are held in thecolumn circuits 140 in the way similar to that in the first exemplaryembodiment, and are subjected to a subtraction process to remove noise.

Through the operation described above, a global electronic shutteroperation is implemented. In this exemplary embodiment, furthermore,each of the pixels 100 includes a sample-and-hold circuit. Thisconfiguration enables a reduction in random noise such as reset noise.

Seventh Exemplary Embodiment

An imaging system according to an exemplary embodiment of the presentinvention will be described. Examples of the imaging system include adigital still camera, a digital camcorder, a video head, a copyingmachine, a facsimile machine, a mobile phone, a vehicle-mountablecamera, and an observation satellite. FIG. 22 is a block diagram of adigital still camera as an example of the imaging system.

Referring to FIG. 22, the digital still camera includes the followingcomponents. A barrier 1001 is configured to protect a lens 1002. Thelens 1002 is configured to form an optical image of an object on aphotoelectric conversion device (or an imaging device) 1004. An aperture1003 is capable of changing the amount of light transmitted through thelens 1002. The photoelectric conversion device 1004 is any of thephotoelectric conversion devices described above in the foregoingexemplary embodiments, and is configured to convert the optical imageformed by the lens 1002 into image data. By way of example, ananalog-to-digital (AD) conversion unit is formed on the semiconductorsubstrate of the photoelectric conversion device 1004. A signalprocessing unit 1007 is configured to perform various kinds ofcorrection on imaging data output from the photoelectric conversiondevice 1004 and to compress the data. A timing generator 1008 outputsvarious timing signals to the photoelectric conversion device 1004 andthe signal processing unit 1007. An overall control unit (or an overallcontrol/arithmetic unit) 1009 is configured to control the overalldigital still camera. A frame memory unit 1010 is configured totemporarily store image data. A recording medium control interface (I/F)unit 1011 is an interface unit for recording or reading data on or froma recording medium 1012. The recording medium 1012 is a removablerecording medium, such as a semiconductor memory, on or from which theimaging data is recorded or read. An external interface (I/F) unit 1013is an interface unit configured to communicate with an external computeror a similar device. The timing signals and the like may be input from adevice outside the imaging system. It may be sufficient that the imagingsystem includes at least the photoelectric conversion device 1004, andthe signal processing unit 1007 to process an imaging signal output fromthe photoelectric conversion device 1004.

In this exemplary embodiment, a configuration has been described inwhich the photoelectric conversion device 1004 and the AD conversionunit are disposed on separate semiconductor substrates. Alternatively,the photoelectric conversion device 1004 and the AD conversion unit maybe formed on the same semiconductor substrate. The photoelectricconversion device 1004 and the signal processing unit 1007 may also beformed on the same semiconductor substrate.

Alternatively, each of the pixels 100 may be configured to include afirst photoelectric conversion unit 101A and a second photoelectricconversion unit 101B. The signal processing unit 1007 may be configuredto process a signal based on the electric charge generated by the firstphotoelectric conversion unit 101A and a signal based on the electriccharge generated by the second photoelectric conversion unit 101B, andto obtain information on the distance from the photoelectric conversiondevice 1004 to the object.

In an imaging system according to an exemplary embodiment, thephotoelectric conversion device 1004 is implemented as the photoelectricconversion device according to any of the first exemplary embodiment tothe sixth exemplary embodiment. In the manner described above, byapplying an exemplary embodiment of the present invention, an imagingsystem may obtain a noise-reduced image.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-156791, filed Jul. 31, 2014, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A photoelectric conversion device comprising: apixel, the pixel including a photoelectric conversion unit, and anamplification unit configured to output a signal generated by thephotoelectric conversion unit, the photoelectric conversion unitincluding a first electrode, a second electrode, a photoelectricconversion layer disposed between the first electrode and the secondelectrode, and an insulating layer disposed between the photoelectricconversion layer and the second electrode, the photoelectric conversionlayer including quantum dots.
 2. The photoelectric conversion deviceaccording to claim 1, wherein the photoelectric conversion layer furtherincludes a member formed of a material different from a material of thequantum dots, and wherein the quantum dots are dispersed in the member.3. The photoelectric conversion device according to claim 2, wherein thematerial of the member has a larger band gap than the material of thequantum dots.
 4. The photoelectric conversion device according to claim2, wherein the material of the quantum dots absorbs light having alonger wavelength than light having a longest wavelength that thematerial of the member absorbs.
 5. The photoelectric conversion deviceaccording to claim 1, wherein the photoelectric conversion layer furtherincludes a cover layer configured to cover the quantum dots, and whereina material of the cover layer has a larger band gap than a material ofthe quantum dots.
 6. The photoelectric conversion device according toclaim 1, wherein the photoelectric conversion layer further includes acover layer configured to cover the quantum dots, and wherein a materialof the quantum dots absorbs light having a longer wavelength than lighthaving a longest wavelength that a material of the cover layer absorbs.7. The photoelectric conversion device according to claim 1, wherein thephotoelectric conversion layer further includes a buried layer in whichthe quantum dots are buried, and wherein a material of the buried layerhas a larger band gap than a material of the quantum dots.
 8. Thephotoelectric conversion device according to claim 1, wherein thephotoelectric conversion layer further includes a buried layer in whichthe quantum dots are buried, and wherein a material of the quantum dotsabsorbs light having a longer wavelength than light having a longestwavelength that a material of the buried layer absorbs.
 9. Thephotoelectric conversion device according to claim 1, wherein thequantum dots include a quantum dot having a particle size in a range of1 nm to 20 nm.
 10. The photoelectric conversion device according toclaim 1, wherein the photoelectric conversion layer further includes ashell configured to surround the quantum dots.
 11. The photoelectricconversion device according to claim 10, wherein a ratio of a latticeconstant of a material of the quantum dots to a lattice constant of amaterial of the shell member is in a range of 0.9 to 1.1.
 12. Thephotoelectric conversion device according to claim 1, wherein thequantum dots is formed of a material selected from the group consistingof PbS, PbSe, PbTe, CdS, CdSe, CdTe, C, Si, and Ge.
 13. A photoelectricconversion device comprising: a pixel, the pixel including aphotoelectric conversion unit, and an amplification unit configured tooutput a signal generated by the photoelectric conversion unit, thephotoelectric conversion unit including a first electrode, a secondelectrode, a photoelectric conversion layer disposed between the firstelectrode and the second electrode, and an insulating layer disposedbetween the photoelectric conversion layer and the second electrode, thephotoelectric conversion layer including a first member, and a pluralityof particles arranged in the first member and having a particle size ina range of 1 nm to 20 nm.
 14. The photoelectric conversion deviceaccording to claim 1, further comprising: a reset unit configured tosupply a reset voltage to the second electrode; a first capacitorincluding a first terminal and a second terminal, the first terminalbeing electrically connected to the second electrode; and a voltagesupply unit configured to supply at least a first voltage and a secondvoltage different from the first voltage to the second terminal, whereinthe amplification unit is electrically connected to the secondelectrode, and wherein the following relationship is satisfied:${\frac{C\; 1}{{C\; 1} + {C\; 2}} > \frac{{Vs} - {Vres}}{{{Vd}\; 2} - {{Vd}\; 1}}},$where Vs denotes a voltage supplied to the first electrode, Vd1 denotesthe first voltage, Vd2 denotes the second voltage, Vres denotes thereset voltage, C1 denotes a capacitance value of the first capacitor,and C2 denotes a capacitance value of a second capacitor formed by thefirst electrode and the second electrode.
 15. The photoelectricconversion device according to claim 14, wherein the first voltage issupplied to the second terminal to accumulate signal charge in thephotoelectric conversion layer, and wherein the second voltage issupplied to the second terminal to discharge the signal charge from thephotoelectric conversion layer.
 16. The photoelectric conversion deviceaccording to claim 1, further comprising: a reset unit configured tosupply a reset voltage to the second electrode; a first capacitorelectrically connected to the second electrode; and a voltage supplyunit configured to supply at least a first voltage and a second voltagedifferent from the first voltage to the first electrode, wherein theamplification unit is electrically connected to the second electrode,and wherein the following relationship is satisfied:${\frac{C\; 1}{C\; 2} > \frac{{Vres} - {{Vs}\; 1}}{{{Vs}\; 2} - {Vres}}},$where Vs1 denotes the first voltage, Vs2 denotes the second voltage,Vres denotes the reset voltage, C1 denotes a capacitance value of thefirst capacitor, and C2 denotes a capacitance value of a secondcapacitor formed by the first electrode and the second electrode. 17.The photoelectric conversion device according to claim 16, wherein thefirst voltage is supplied to the first electrode to accumulate signalcharge in the photoelectric conversion layer, and wherein the secondvoltage is supplied to the first electrode to discharge the signalcharge from the photoelectric conversion layer.
 18. The photoelectricconversion device according to claim 14, wherein the first capacitorincludes two electrodes facing each other.
 19. An imaging systemcomprising: the photoelectric conversion device according to claim 1;and a signal processing device configured to process a signal from thephotoelectric conversion device.
 20. The imaging system according toclaim 19, wherein two photoelectric conversion units, each comprisingthe photoelectric conversion unit, are disposed for each pixel, andwherein the signal processing device processes a signal based onelectric charge generated by the two photoelectric conversion units, andobtains information on a distance from the photoelectric conversiondevice to an object.